Some experience with the igc branch

Some experience with the igc branch

[Óscar Fuentes]

I've using the igc branch for the past weeks. It was mostly Dart/Flutter
development with lsp-dart / lsp-mode enabled, with all its default
features enabled. On top of that, I use the flx completion algorithm.

This setup puts a lot of stress on GC. To illustrate, on master Emacs
after setting garbage-collection-messages to t, one can see that simply
writing a few characters triggers GC several times, each with its
corresponding pause, which may be very noticeable ("uh! that keypress
didn't register... wait, there it is.") The experience is not great.
Quite miserable, I would say. People suggest playing with
gc-cons-threshold (I have mine set to 10'000'000) but those tricks
simply make things a bit less awful.

With igc the pauses are still there, but they much shorter and
predictable, they no longer distract me from thinking on what I'm
writing, which is a huge improvement. I suspect that some of those
pauses are not related to garbage collection (executing code and moving
data also takes time.)

TL/DR: now I enjoy using Emacs with this setup and I'm no longer tempted
to switch to other editors for this type of work.

A big thank you to all involved on this feature.

Re: Some experience with the igc branch

[Gerd Möllmann]

Óscar Fuentes <ofv@wanadoo.es> writes:

> I've using the igc branch for the past weeks. It was mostly Dart/Flutter
> development with lsp-dart / lsp-mode enabled, with all its default
> features enabled. On top of that, I use the flx completion algorithm.
>
> This setup puts a lot of stress on GC. To illustrate, on master Emacs
> after setting garbage-collection-messages to t, one can see that simply
> writing a few characters triggers GC several times, each with its
> corresponding pause, which may be very noticeable ("uh! that keypress
> didn't register... wait, there it is.") The experience is not great.
> Quite miserable, I would say. People suggest playing with
> gc-cons-threshold (I have mine set to 10'000'000) but those tricks
> simply make things a bit less awful.
>
> With igc the pauses are still there, but they much shorter and
> predictable, they no longer distract me from thinking on what I'm
> writing, which is a huge improvement. I suspect that some of those
> pauses are not related to garbage collection (executing code and moving
> data also takes time.)
>
> TL/DR: now I enjoy using Emacs with this setup and I'm no longer tempted
> to switch to other editors for this type of work.
>
> A big thank you to all involved on this feature.

👍 

Re: Some experience with the igc branch

[Gerd Möllmann]

Óscar Fuentes <ofv@wanadoo.es> writes:

> With igc the pauses are still there, but they much shorter and
> predictable, they no longer distract me from thinking on what I'm
> writing, which is a huge improvement. I suspect that some of those
> pauses are not related to garbage collection (executing code and moving
> data also takes time.)

In my case, with Eglot, the following settings made a difference;

  :custom
  (eglot-sync-connect nil)
  (eglot-events-buffer-config '(:size 0 :format full)))

Don't know if Lsp-mode has similar knobs.

Re: Some experience with the igc branch

[Pip Cet via Emacs development discussions.]

Óscar Fuentes <ofv@wanadoo.es> writes:
> With igc the pauses are still there, but they much shorter and
> predictable, they no longer distract me from thinking on what I'm
> writing, which is a huge improvement. I suspect that some of those
> pauses are not related to garbage collection (executing code and moving
> data also takes time.)

Quite possible.  Even if it is GC, please keep in mind that MPS has many
settings which you can play with, and it can improve things a lot.  It's
not too early to become a fan of the scratch/igc branch, but it is too
early to reject it for performance reasons.  It's a "heads you lose, tails I
win" situation, I guess.

> TL/DR: now I enjoy using Emacs with this setup and I'm no longer tempted
> to switch to other editors for this type of work.

I think this is an important point: ultimately, it's about having daily
drivers.  We need to remove the remaining impediments for that:

1. The signal issue.  I don't have a good way to fix this and make
everyone happy, but I do have a solution which hasn't caused a crash for
me in quite a while.  It may be good enough.

2. no-purespace.  Merging that into scratch/igc would help, well, me.
What do others think?

3. bytecode stack marking.  That comment raises my red-flag alert,
because it sounds like we're just accepting a preventable crash at this
stage rather than wanting to do anything about it.  The reality, of
course, is different, but I'd be happier if we refused to create a byte
code object that intends to use more stack than we can guarantee we
would scan.  Can we do that?

Pip

Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> 3. bytecode stack marking.  That comment raises my red-flag alert,
> because it sounds like we're just accepting a preventable crash at this
> stage rather than wanting to do anything about it.  The reality, of
> course, is different, but I'd be happier if we refused to create a byte
> code object that intends to use more stack than we can guarantee we
> would scan.  Can we do that?
>
> Pip

You mean my comment here?

static mps_res_t
scan_bc (mps_ss_t ss, void *start, void *end, void *closure)
{
  MPS_SCAN_BEGIN (ss)
  {
    struct igc_thread_list *t = closure;
    struct bc_thread_state *bc = &t->d.ts->bc;
    igc_assert (start == (void *) bc->stack);
    igc_assert (end == (void *) bc->stack_end);
    /* FIXME/igc: AFAIU the current top frame starts at
       bc->fp->next_stack and has a maximum length that is given by the
       bytecode being executed (COMPILED_STACK_DEPTH). So, we need to
       scan upto bc->fo->next_stack + that max depth to be safe.  Since
       I don't have that number ATM, I'm using an arbitrary estimate for
       now.

       This must be changed to something better. Note that Mattias said
       the bc stack marking will be changed in the future.  */
    const size_t HORRIBLE_ESTIMATE = 1024;
    char *scan_end = bc_next_frame (bc->fp);
    scan_end += HORRIBLE_ESTIMATE;
    end = min (end, (void *) scan_end);
    if (end > start)
      IGC_FIX_CALL (ss, scan_ambig (ss, start, end, NULL));
  }
  MPS_SCAN_END (ss);
  return MPS_RES_OK;
}

I never felt like changing the byte code stack, TBH. For reasons :-).

Re: Some experience with the igc branch

[Óscar Fuentes]

Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
writes:

>> I suspect that some of those
>> pauses are not related to garbage collection (executing code and moving
>> data also takes time.)
>
> Quite possible.  Even if it is GC, please keep in mind that MPS has many
> settings which you can play with, and it can improve things a lot.  It's
> not too early to become a fan of the scratch/igc branch, but it is too
> early to reject it for performance reasons.  It's a "heads you lose, tails I
> win" situation, I guess.

IIRC MPS is well documented and I can look up those settings, but does
Emacs collect the required info for taking informed decisions?

Anyway, with the setup I'm using for this job is totally unrealistic to
expect instant reaction from Emacs, there is too much heavy stuff
kicking in for every keypress.

> 1. The signal issue.  I don't have a good way to fix this and make
> everyone happy, but I do have a solution which hasn't caused a crash for
> me in quite a while.  It may be good enough.

Inevitably, a few minutes after sending my message Emacs froze after
working flawlessly since you fixed the JSON issue.

Redisplay just stopped while showing the menu, no crash nor infinite
loop, its CPU usage was typical for the repeating timers that my config
creates. Sadly, instead of attaching gdb I tried to wake up Emacs by
sending SIGUSR1 (no effect, as it is the wrong signal, should be
SIGUSR2) and then sent SINGINT by mistake, which terminated the process.

It's very likely that MPS is innocent on this, but I'm happy to apply
and test any stability improvement patch you have and wish to share.

Thanks.

Re: Some experience with the igc branch

[Helmut Eller]

On Sun, Dec 22 2024, Pip Cet wrote:

> 2. no-purespace.  Merging that into scratch/igc would help, well, me.
> What do others think?

No objections from me.

> 3. bytecode stack marking.  That comment raises my red-flag alert,
> because it sounds like we're just accepting a preventable crash at this
> stage rather than wanting to do anything about it.  The reality, of
> course, is different, but I'd be happier if we refused to create a byte
> code object that intends to use more stack than we can guarantee we
> would scan.  Can we do that?

Maybe the bytecode engine could handle large stack frames differently
from small stack frames.

For large stack frames we would:
 1. initialize the stack frame with NULLs
 2. bump the stack pointer
 3. now the stack frame is usable

For small stack frames, we would skip step 1 but the GC would always
scan one extra "small frame with maximal length".

Helmut

Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> Óscar Fuentes <ofv@wanadoo.es> writes:
>> With igc the pauses are still there, but they much shorter and
>> predictable, they no longer distract me from thinking on what I'm
>> writing, which is a huge improvement. I suspect that some of those
>> pauses are not related to garbage collection (executing code and moving
>> data also takes time.)
>
> Quite possible.  Even if it is GC, please keep in mind that MPS has many
> settings which you can play with, and it can improve things a lot.  It's
> not too early to become a fan of the scratch/igc branch, but it is too
> early to reject it for performance reasons.  It's a "heads you lose, tails I
> win" situation, I guess.
>
>> TL/DR: now I enjoy using Emacs with this setup and I'm no longer tempted
>> to switch to other editors for this type of work.
>
> I think this is an important point: ultimately, it's about having daily
> drivers.  We need to remove the remaining impediments for that:
>
> 1. The signal issue.  I don't have a good way to fix this and make
> everyone happy, but I do have a solution which hasn't caused a crash for
> me in quite a while.  It may be good enough.

TBH, I'd have put it in already.

> 2. no-purespace.  Merging that into scratch/igc would help, well, me.
> What do others think?

Doesn't affect me much.

Re: Some experience with the igc branch

[Pip Cet via Emacs development discussions.]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Pip Cet <pipcet@protonmail.com> writes:
>
>> Óscar Fuentes <ofv@wanadoo.es> writes:
>>> With igc the pauses are still there, but they much shorter and
>>> predictable, they no longer distract me from thinking on what I'm
>>> writing, which is a huge improvement. I suspect that some of those
>>> pauses are not related to garbage collection (executing code and moving
>>> data also takes time.)
>>
>> Quite possible.  Even if it is GC, please keep in mind that MPS has many
>> settings which you can play with, and it can improve things a lot.  It's
>> not too early to become a fan of the scratch/igc branch, but it is too
>> early to reject it for performance reasons.  It's a "heads you lose, tails I
>> win" situation, I guess.
>>
>>> TL/DR: now I enjoy using Emacs with this setup and I'm no longer tempted
>>> to switch to other editors for this type of work.
>>
>> I think this is an important point: ultimately, it's about having daily
>> drivers.  We need to remove the remaining impediments for that:
>>
>> 1. The signal issue.  I don't have a good way to fix this and make
>> everyone happy, but I do have a solution which hasn't caused a crash for
>> me in quite a while.  It may be good enough.
>
> TBH, I'd have put it in already.

Pushed it now.  It is imperfect, but better than crashing.

>> 2. no-purespace.  Merging that into scratch/igc would help, well, me.
>> What do others think?
>
> Doesn't affect me much.

Well, it does cause some noise, so I thought I'd ask first.

Pip

Re: Some experience with the igc branch

[Pip Cet via Emacs development discussions.]

Óscar Fuentes <ofv@wanadoo.es> writes:

> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> writes:
>
>>> I suspect that some of those
>>> pauses are not related to garbage collection (executing code and moving
>>> data also takes time.)
>>
>> Quite possible.  Even if it is GC, please keep in mind that MPS has many
>> settings which you can play with, and it can improve things a lot.  It's
>> not too early to become a fan of the scratch/igc branch, but it is too
>> early to reject it for performance reasons.  It's a "heads you lose, tails I
>> win" situation, I guess.
>
> IIRC MPS is well documented and I can look up those settings, but does
> Emacs collect the required info for taking informed decisions?

Not that I'm aware of, at this point.

>> 1. The signal issue.  I don't have a good way to fix this and make
>> everyone happy, but I do have a solution which hasn't caused a crash for
>> me in quite a while.  It may be good enough.
>
> Inevitably, a few minutes after sending my message Emacs froze after
> working flawlessly since you fixed the JSON issue.

Sorry to hear it, and thanks for letting us know!  If it happens again,
any additional information you can provide would be very helpful.

> Redisplay just stopped while showing the menu, no crash nor infinite
> loop, its CPU usage was typical for the repeating timers that my config
> creates.

That's a bit odd.  It might be the signal issue, but that's purely a
guess.  If it happens again, please let us know.

Which windowing system are you using, and how are you displaying menus,
though?

> It's very likely that MPS is innocent on this, but I'm happy to apply
> and test any stability improvement patch you have and wish to share.

I just pushed the temporary fix for the signal issue, which should
improve stability.

Pip

Re: Some experience with the igc branch

[Óscar Fuentes]

Pip Cet <pipcet@protonmail.com> writes:

>> Redisplay just stopped while showing the menu, no crash nor infinite
>> loop, its CPU usage was typical for the repeating timers that my config
>> creates.
>
> That's a bit odd.  It might be the signal issue, but that's purely a
> guess.  If it happens again, please let us know.

Sure.

> Which windowing system are you using, and how are you displaying menus,
> though?

Configured using:
 'configure CPPFLAGS=-I/home/oscar/dev/include/mps
 LDFLAGS=-L/home/oscar/dev/other/mps/code --with-native-compilation
 --with-tree-sitter --without-toolkit-scroll-bars --with-x-toolkit=lucid
 --with-modules --without-imagemagick --with-mps=yes'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG LIBOTF
LIBSELINUX LIBXML2 MODULES MPS NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
SECCOMP SOUND SQLITE3 THREADS TIFF TREE_SITTER WEBP X11 XAW3D XDBE XIM
XINPUT2 XPM LUCID ZLIB

I have the menubar disabled (menu-bar-mode -1) and use a custom command
to open it:

(defun my-menu-bar-open-after ()
  (remove-hook 'pre-command-hook 'my-menu-bar-open-after)
  (when (eq menu-bar-mode 42)
    (menu-bar-mode -1)))

(defun my-menu-bar-open (&rest args)
  (interactive)
  (let ((open menu-bar-mode))
    (unless open
      (menu-bar-mode 1))
    (funcall 'menu-bar-open args)
    (unless open
      (setq menu-bar-mode 42)
      (add-hook 'pre-command-hook 'my-menu-bar-open-after))))

(global-set-key [f10] 'my-menu-bar-open)

On that same session I used the command multiple times.

>> It's very likely that MPS is innocent on this, but I'm happy to apply
>> and test any stability improvement patch you have and wish to share.
>
> I just pushed the temporary fix for the signal issue, which should
> improve stability.

Emacs is already running here with that commit. Thanks!

Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Pip Cet <pipcet@protonmail.com> writes:
>>
>>> Óscar Fuentes <ofv@wanadoo.es> writes:
>>>> With igc the pauses are still there, but they much shorter and
>>>> predictable, they no longer distract me from thinking on what I'm
>>>> writing, which is a huge improvement. I suspect that some of those
>>>> pauses are not related to garbage collection (executing code and moving
>>>> data also takes time.)
>>>
>>> Quite possible.  Even if it is GC, please keep in mind that MPS has many
>>> settings which you can play with, and it can improve things a lot.  It's
>>> not too early to become a fan of the scratch/igc branch, but it is too
>>> early to reject it for performance reasons.  It's a "heads you lose, tails I
>>> win" situation, I guess.
>>>
>>>> TL/DR: now I enjoy using Emacs with this setup and I'm no longer tempted
>>>> to switch to other editors for this type of work.
>>>
>>> I think this is an important point: ultimately, it's about having daily
>>> drivers.  We need to remove the remaining impediments for that:
>>>
>>> 1. The signal issue.  I don't have a good way to fix this and make
>>> everyone happy, but I do have a solution which hasn't caused a crash for
>>> me in quite a while.  It may be good enough.
>>
>> TBH, I'd have put it in already.
>
> Pushed it now.  It is imperfect, but better than crashing.

100%. Thanks!

>
>>> 2. no-purespace.  Merging that into scratch/igc would help, well, me.
>>> What do others think?
>>
>> Doesn't affect me much.
>
> Well, it does cause some noise, so I thought I'd ask first.
>
> Pip

That's nice of you.

Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> Óscar Fuentes <ofv@wanadoo.es> writes:
>
>> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>> writes:
>>
>>>> I suspect that some of those
>>>> pauses are not related to garbage collection (executing code and moving
>>>> data also takes time.)
>>>
>>> Quite possible.  Even if it is GC, please keep in mind that MPS has many
>>> settings which you can play with, and it can improve things a lot.  It's
>>> not too early to become a fan of the scratch/igc branch, but it is too
>>> early to reject it for performance reasons.  It's a "heads you lose, tails I
>>> win" situation, I guess.
>>
>> IIRC MPS is well documented and I can look up those settings, but does
>> Emacs collect the required info for taking informed decisions?
>
> Not that I'm aware of, at this point.

Me neither.

(And, at least for me personally, "interactive performance", i.e. the
impression a user gets when he's using Emacs interactively, is the only
interesting part. That's difficult to measure of course. I don't care
much about performance improvements I don't notice :-)).

>
>>> 1. The signal issue.  I don't have a good way to fix this and make
>>> everyone happy, but I do have a solution which hasn't caused a crash for
>>> me in quite a while.  It may be good enough.
>>
>> Inevitably, a few minutes after sending my message Emacs froze after
>> working flawlessly since you fixed the JSON issue.
>
> Sorry to hear it, and thanks for letting us know!  If it happens again,
> any additional information you can provide would be very helpful.
>
>> Redisplay just stopped while showing the menu, no crash nor infinite
>> loop, its CPU usage was typical for the repeating timers that my config
>> creates.
>
> That's a bit odd.  It might be the signal issue, but that's purely a
> guess.  If it happens again, please let us know.
>
> Which windowing system are you using, and how are you displaying menus,
> though?

Yes. I don't think I've ever seen a freeze caused by igc here. It always
was crashes. But one never knows, of course.

Re: Some experience with the igc branch

[Jean Louis]

* Óscar Fuentes <ofv@wanadoo.es> [2024-12-22 22:13]:
> > 1. The signal issue.  I don't have a good way to fix this and make
> > everyone happy, but I do have a solution which hasn't caused a crash for
> > me in quite a while.  It may be good enough.
> 
> Inevitably, a few minutes after sending my message Emacs froze after
> working flawlessly since you fixed the JSON issue.
> 
> Redisplay just stopped while showing the menu, no crash nor infinite
> loop, its CPU usage was typical for the repeating timers that my config
> creates. Sadly, instead of attaching gdb I tried to wake up Emacs by
> sending SIGUSR1 (no effect, as it is the wrong signal, should be
> SIGUSR2) and then sent SINGINT by mistake, which terminated the process.
> 
> It's very likely that MPS is innocent on this, but I'm happy to apply
> and test any stability improvement patch you have and wish to share.

I was using that branch for longer, but being heavy daily user of
Emacs with serious business, I cannot use it, it is not yet
stable. Reasons I have sent already to this list, I had no issues
since I switched to standard Emacs.

Most terrible was that ghostly appearance of words and characters
which I didn't type, or totally scrambling characters which I type.


-- 
Jean Louis

Re: Some experience with the igc branch

[Eli Zaretskii]

> Date: Sun, 22 Dec 2024 22:26:11 +0000
> Cc: Óscar Fuentes <ofv@wanadoo.es>, emacs-devel@gnu.org,
>  Helmut Eller <eller.helmut@gmail.com>, Andrea Corallo <acorallo@gnu.org>
> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> 
> >> 1. The signal issue.  I don't have a good way to fix this and make
> >> everyone happy, but I do have a solution which hasn't caused a crash for
> >> me in quite a while.  It may be good enough.
> >
> > TBH, I'd have put it in already.
> 
> Pushed it now.  It is imperfect, but better than crashing.

Why didn't we discuss this with MPS folks?  A program can legitimately
call some code from a signal handler, so the limitations that MPS
seems to impose now are not very reasonable.  Maybe we are missing
some feature, or maybe the MPS folks will agree to extend the library
to provide better support for programs that use signals.  E.g., AFAIU
with this code installed, we are limiting our profiler too much (it
will never report GC, IIRC?).  I think igc_busy_p returns non-zero in
too many situations where delivering signals could not possibly cause
harm, like during object allocation, AFAIR.  According to
documentation, that function is not intended for this kind of purpose.

IOW, we had discussions about this which never concluded anything, and
we should pick up where we left off and solve this problem.

We should definitely try improving this before we land the branch on
master.  We shouldn't consider this solution "good enough", but just a
temporary kludge meant to avoid too frequent crashes.

Discussion with MPS people

[Gerd Möllmann]

Eli Zaretskii <eliz@gnu.org> writes:

> Why didn't we discuss this with MPS folks?

That would be a good thing. Maybe we can get at least some initial
contact going.

I've CC'd Richard Brooksby, who is one of the main people behind MPS.
(AFAIU; sorry Richard if I under-represent your role.) I've seen that he
recently answered on the bug list, so maybe he's interested in helping
us.

@Richard:

Eli is an Emacs co-maintainer, and this mail goes in CC to the
emacs-devel mailing list. Pip Cet has taken over further development of
the branch scratch/igc, which contains a GC for Emacs based on MPS.
Helmut Eller is basically the third person doing importatnt work on
scratch/igc in the past.

Re: Discussion with MPS people

[Gerd Möllmann]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>> Why didn't we discuss this with MPS folks?
>
> That would be a good thing. Maybe we can get at least some initial
> contact going.
>
> I've CC'd Richard Brooksby, who is one of the main people behind MPS.
> (AFAIU; sorry Richard if I under-represent your role.) I've seen that he
> recently answered on the bug list, so maybe he's interested in helping
> us.
>
> @Richard:
>
> Eli is an Emacs co-maintainer, and this mail goes in CC to the
> emacs-devel mailing list. Pip Cet has taken over further development of
> the branch scratch/igc, which contains a GC for Emacs based on MPS.
> Helmut Eller is basically the third person doing importatnt work on
> scratch/igc in the past.

And of course I've forgotten to actually add Richard in CC. Now done.

Re: Make Signal handling patch platform-dependent?

[Pip Cet via Emacs development discussions.]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Pip Cet <pipcet@protonmail.com> writes:
>
>> And, who knows, using a separate thread might help (debugging, not
>> performance).
>
> Yeah, more long-term goals, I'd guess. I'm glad we're moving forward,
> ATM :-).

If we come up with a solution to the signal issue which works but
requires the creation of extra threads, would that prevent the merge?

>> The rest of this email is about a half-baked idea to perform dry-run
>> background GCs to facilitate debugging.  It's tantalizingly close to
>> offering performance benefits, but doesn't quite get there, and it
>> doesn't have to: it'd help us detect leaked references to objects, and
>> that's all it needs to do.
>>
>> I'm still thinking about double-mapping MPS segments so one thread can
>> scan them using a "privileged" mapping while the barrier is in place for
>> the ordinary mapping and prevents access to that segment.
>
> Quick question upfront; I'll have to think longer about the rest, and
> maybe try to find existing examples: the double-mapping. How would that
> be done? I know about page-table manipulation, but I don't think it's
> easily doable, at least not on macOS. What would you use for
> double-mapping?

My understanding is the two options are SysV shm* (clunky) or mmapping a
file handle corresponding to an already-deleted file, twice (some risk
the OS will synchronize the file to disk, maybe even page it out; also
might count towards the disk quota).

I prefer the latter because I wouldn't actually delete the file, which
would give us a snapshot of the MPS heap in the event of a crash.  If
that isn't enough, we could explicitly snapshot the file once in a
while, before moving objects, giving us the ability to detect where an
object moved to.  (If THAT isn't enough, we'd have two additional
options: either hack MPS not to reuse virtual addresses unless it really
has to, or store the file on a fully journaled file system allowing us
to time-travel through the MPS heap.)  Also, I've never used shm*.

If there's a third option, it'd be great to learn about it.

Needless to say, double-mapping doubles the VM size, which is limited on
32-bit systems.

I don't think virtually-indexed caches are a thing anymore (if the cache
doesn't recognize two VAs correspond to the same PA, well, great fun
ensues).

IIUC, some aarch64 systems, but not those usually running macOS, have
weak cache coherency, and as double-mapping is a valid but rare thing to
do, who knows what would happen.

Pip

Re: Make Signal handling patch platform-dependent?

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Pip Cet <pipcet@protonmail.com> writes:
>>
>>> And, who knows, using a separate thread might help (debugging, not
>>> performance).
>>
>> Yeah, more long-term goals, I'd guess. I'm glad we're moving forward,
>> ATM :-).
>
> If we come up with a solution to the signal issue which works but
> requires the creation of extra threads, would that prevent the merge?

I think that's for Eli to answer.

>>> The rest of this email is about a half-baked idea to perform dry-run
>>> background GCs to facilitate debugging.  It's tantalizingly close to
>>> offering performance benefits, but doesn't quite get there, and it
>>> doesn't have to: it'd help us detect leaked references to objects, and
>>> that's all it needs to do.
>>>
>>> I'm still thinking about double-mapping MPS segments so one thread can
>>> scan them using a "privileged" mapping while the barrier is in place for
>>> the ordinary mapping and prevents access to that segment.
>>
>> Quick question upfront; I'll have to think longer about the rest, and
>> maybe try to find existing examples: the double-mapping. How would that
>> be done? I know about page-table manipulation, but I don't think it's
>> easily doable, at least not on macOS. What would you use for
>> double-mapping?
>
> My understanding is the two options are SysV shm* (clunky) or mmapping a
> file handle corresponding to an already-deleted file, twice (some risk
> the OS will synchronize the file to disk, maybe even page it out; also
> might count towards the disk quota).
>
> I prefer the latter because I wouldn't actually delete the file, which
> would give us a snapshot of the MPS heap in the event of a crash.  If
> that isn't enough, we could explicitly snapshot the file once in a
> while, before moving objects, giving us the ability to detect where an
> object moved to.  (If THAT isn't enough, we'd have two additional
> options: either hack MPS not to reuse virtual addresses unless it really
> has to, or store the file on a fully journaled file system allowing us
> to time-travel through the MPS heap.)  Also, I've never used shm*.
>
> If there's a third option, it'd be great to learn about it.

I don't know of a third option.

>
> Needless to say, double-mapping doubles the VM size, which is limited on
> 32-bit systems.
>
> I don't think virtually-indexed caches are a thing anymore (if the cache
> doesn't recognize two VAs correspond to the same PA, well, great fun
> ensues).
>
> IIUC, some aarch64 systems, but not those usually running macOS, have
> weak cache coherency, and as double-mapping is a valid but rare thing to
> do, who knows what would happen.
>
> Pip

Thanks so far!

Re: Some experience with the igc branch

[Pip Cet via Emacs development discussions.]

"Eli Zaretskii" <eliz@gnu.org> writes:

>> Date: Sun, 22 Dec 2024 22:26:11 +0000
>> Cc: Óscar Fuentes <ofv@wanadoo.es>, emacs-devel@gnu.org,
>>  Helmut Eller <eller.helmut@gmail.com>, Andrea Corallo <acorallo@gnu.org>
>> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>>
>> >> 1. The signal issue.  I don't have a good way to fix this and make
>> >> everyone happy, but I do have a solution which hasn't caused a crash for
>> >> me in quite a while.  It may be good enough.
>> >
>> > TBH, I'd have put it in already.
>>
>> Pushed it now.  It is imperfect, but better than crashing.
>
> Why didn't we discuss this with MPS folks?  A program can legitimately

Because...

> call some code from a signal handler, so the limitations that MPS
> seems to impose now are not very reasonable.  Maybe we are missing

...if they were interested, maybe they've read this or some other
blanket accusation of being "unreasonable", and became uninterested
quickly.  I know I would.

> some feature, or maybe the MPS folks will agree to extend the library
> to provide better support for programs that use signals.  E.g., AFAIU
> with this code installed, we are limiting our profiler too much (it
> will never report GC, IIRC?).  I think igc_busy_p returns non-zero in
> too many situations where delivering signals could not possibly cause
> harm, like during object allocation, AFAIR.  According to
> documentation, that function is not intended for this kind of purpose.
>
> IOW, we had discussions about this which never concluded anything, and
> we should pick up where we left off and solve this problem.

I have a different idea using a separate allocation thread (for the slow
path only, of course).  Would that be potentially acceptable?

It would limit MPS to systems providing a working atomic.h header, and
in practice also require some sort of working (and reasonably fast)
inter-thread signalling (though I suspect it'd be faster to run both
threads on the same core, since it's a handover rather than a
parallelism situation).  That excludes very few systems these days
(sorry, MS-DOS).

I'll spare you most of the details for now, but having read the mps
header, MPS allocation is not safe to use from separate threads without
locking the AP (or having per-thread APs), which we might end up doing
on Windows, IIRC.  I'd rather give those (potential) issues a wide
berth.  Also, by the campsite rule, merging MPS shouldn't make it harder
to move in the direction of multi-threaded Emacs.

Better debugging (which I agree with you is something we need to
improve), no MPS modification.  Performance implications TBD.

> We should definitely try improving this before we land the branch on
> master.  We shouldn't consider this solution "good enough", but just a
> temporary kludge meant to avoid too frequent crashes.

Agreed.

Pip

Re: Make Signal handling patch platform-dependent?

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Óscar Fuentes <ofv@wanadoo.es>,  emacs-devel@gnu.org,
>  Helmut Eller <eller.helmut@gmail.com>,  Andrea Corallo <acorallo@gnu.org>
> Date: Mon, 23 Dec 2024 15:54:36 +0100
> 
> Pip Cet <pipcet@protonmail.com> writes:
> 
> > If we come up with a solution to the signal issue which works but
> > requires the creation of extra threads, would that prevent the merge?
> 
> I think that's for Eli to answer.

I don't see why extra threads would be a problem, as long as they
don't use the Lisp machine or any parts of the global state.  We
already have several threads on MS-Windows, including for emulating
Posix signals, so I don't see why adding C threads on Posix systems
would be "verboten".

Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> I'll spare you most of the details for now, but having read the mps
> header, MPS allocation is not safe to use from separate threads without
> locking the AP (or having per-thread APs), which we might end up doing
> on Windows, IIRC.

Now I'm confused. We're using thread allocation points. See
create_thread_aps, thread_ap, and so on. 

Re: Some experience with the igc branch

[Pip Cet via Emacs development discussions.]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Pip Cet <pipcet@protonmail.com> writes:
>
>> I'll spare you most of the details for now, but having read the mps
>> header, MPS allocation is not safe to use from separate threads without
>> locking the AP (or having per-thread APs), which we might end up doing
>> on Windows, IIRC.
>
> Now I'm confused. We're using thread allocation points. See
> create_thread_aps, thread_ap, and so on.

I was confused.  This is only a problem if we allocate memory from a
signal handler, which is effectively sharing the per-thread structure.

(I'm still confused. My patch worked on the first attempt, which my code
never does.  I suspect that while I made a mistake, it caused a subtle
bug rather than an obvious one.)

And we don't want to allocate memory from signal handlers, right?  We
could, now (see warnings below):

diff --git a/src/igc.c b/src/igc.c
index eb72406e529..14ecc30f982 100644
--- a/src/igc.c
+++ b/src/igc.c
@@ -747,19 +747,41 @@ IGC_DEFINE_LIST (igc_root);
 
 /* Registry entry for an MPS thread mps_thr_t.  */
 
+#include <pthread.h>
+#include <stdatomic.h>
+
+struct emacs_ap
+{
+  mps_ap_t mps_ap;
+  struct igc *gc;
+  pthread_t allocation_thread;
+  atomic_uintptr_t usable_memory;
+  atomic_uintptr_t usable_bytes;
+
+  atomic_uintptr_t waiting_threads;
+  atomic_uintptr_t requested_bytes;
+  atomic_intptr_t requested_type;
+};
+
+typedef struct emacs_ap emacs_ap_t;
+
+#ifndef ATOMIC_POINTER_LOCK_FREE
+#error "this probably won't work"
+#endif
+
 struct igc_thread
 {
   struct igc *gc;
   mps_thr_t thr;
 
   /* Allocation points for the thread.  */
-  mps_ap_t dflt_ap;
-  mps_ap_t leaf_ap;
-  mps_ap_t weak_strong_ap;
-  mps_ap_t weak_weak_ap;
-  mps_ap_t weak_hash_strong_ap;
-  mps_ap_t weak_hash_weak_ap;
-  mps_ap_t immovable_ap;
+  emacs_ap_t dflt_ap;
+  emacs_ap_t leaf_ap;
+  emacs_ap_t weak_strong_ap;
+  emacs_ap_t weak_weak_ap;
+  emacs_ap_t weak_hash_strong_ap;
+  emacs_ap_t weak_hash_weak_ap;
+  emacs_ap_t immovable_ap;
 
   /* Quick access to the roots used for specpdl, bytecode stack and
      control stack.  */
@@ -805,6 +827,8 @@ IGC_DEFINE_LIST (igc_thread);
 
   /* Registered threads.  */
   struct igc_thread_list *threads;
+
+  pthread_cond_t cond;
 };
 
 static bool process_one_message (struct igc *gc);
@@ -2904,8 +2928,84 @@ igc_root_destroy_comp_unit_eph (struct Lisp_Native_Comp_Unit *u)
   maybe_destroy_root (&u->data_eph_relocs_root);
 }
 
+static mps_addr_t alloc_impl_raw (size_t size, enum igc_obj_type type, mps_ap_t ap);
+static mps_addr_t alloc_impl (size_t size, enum igc_obj_type type, emacs_ap_t *ap);
+
+static void *igc_allocation_thread (void *ap_v)
+{
+  emacs_ap_t *ap = ap_v;
+  while (true)
+    {
+      if (ap->requested_bytes)
+	{
+	  void *p = alloc_impl_raw (ap->requested_bytes, (enum igc_obj_type) ap->requested_type, ap->mps_ap);
+	  atomic_store (&ap->usable_memory, (uintptr_t) p);
+	  atomic_store (&ap->usable_bytes, ap->requested_bytes);
+	  atomic_store (&ap->requested_type, -1);
+	  atomic_store (&ap->requested_bytes, 0);
+	}
+    }
+
+  return NULL;
+}
+
+static mps_addr_t alloc_impl (size_t size, enum igc_obj_type type, emacs_ap_t *ap)
+{
+  if (size == 0)
+    return 0;
+  while (true)
+    {
+      uintptr_t other_threads = atomic_fetch_add (&ap->waiting_threads, 1);
+      if (other_threads != 0)
+	{
+	  /* we know that the other "thread" is actually on top of us,
+	   * and we're a signal handler. Wait, should we even be
+	   * allocating memory?  We should still eassert that we're the
+	   * right thread. */
+	  emacs_ap_t saved_state;
+	  while (ap->requested_bytes);
+	  memcpy (&saved_state, ap, sizeof saved_state);
+	  atomic_store (&ap->waiting_threads, 0);
+	  mps_addr_t ret = alloc_impl (size, type, ap);
+	  atomic_store (&ap->waiting_threads, saved_state.waiting_threads);
+	  memcpy (ap, &saved_state, sizeof saved_state);
+	  atomic_fetch_add (&ap->waiting_threads, -1);
+	  return ret;
+	}
+
+      atomic_store (&ap->requested_type, (uintptr_t) type);
+      atomic_store (&ap->requested_bytes, (uintptr_t) size);
+
+      while (ap->requested_bytes);
+
+      mps_addr_t ret = (mps_addr_t) ap->usable_memory;
+      atomic_fetch_add (&ap->waiting_threads, -1);
+      return ret;
+    }
+}
+
+static mps_res_t emacs_ap_create_k (emacs_ap_t *ap, mps_pool_t pool,
+				    mps_arg_s *args)
+{
+  atomic_store(&ap->usable_memory, 0);
+  atomic_store(&ap->usable_bytes, 0);
+  atomic_store(&ap->waiting_threads, 0);
+  atomic_store(&ap->requested_bytes, 0);
+
+  pthread_attr_t thread_attr;
+  pthread_attr_init (&thread_attr);
+  pthread_create(&ap->allocation_thread, &thread_attr, igc_allocation_thread, ap);
+
+  return mps_ap_create_k (&ap->mps_ap, pool, args);
+}
+
+static void emacs_ap_destroy (emacs_ap_t *ap)
+{
+  return;
+}
+
 static mps_res_t
-create_weak_ap (mps_ap_t *ap, struct igc_thread *t, bool weak)
+create_weak_ap (emacs_ap_t *ap, struct igc_thread *t, bool weak)
 {
   struct igc *gc = t->gc;
   mps_res_t res;
@@ -2914,14 +3014,14 @@ create_weak_ap (mps_ap_t *ap, struct igc_thread *t, bool weak)
   {
     MPS_ARGS_ADD (args, MPS_KEY_RANK,
 		  weak ? mps_rank_weak () : mps_rank_exact ());
-    res = mps_ap_create_k (ap, pool, args);
+    res = emacs_ap_create_k (ap, pool, args);
   }
   MPS_ARGS_END (args);
   return res;
 }
 
 static mps_res_t
-create_weak_hash_ap (mps_ap_t *ap, struct igc_thread *t, bool weak)
+create_weak_hash_ap (emacs_ap_t *ap, struct igc_thread *t, bool weak)
 {
   struct igc *gc = t->gc;
   mps_res_t res;
@@ -2930,7 +3030,7 @@ create_weak_hash_ap (mps_ap_t *ap, struct igc_thread *t, bool weak)
   {
     MPS_ARGS_ADD (args, MPS_KEY_RANK,
 		  weak ? mps_rank_weak () : mps_rank_exact ());
-    res = mps_ap_create_k (ap, pool, args);
+    res = emacs_ap_create_k (ap, pool, args);
   }
   MPS_ARGS_END (args);
   return res;
@@ -2940,12 +3040,15 @@ create_weak_hash_ap (mps_ap_t *ap, struct igc_thread *t, bool weak)
 create_thread_aps (struct igc_thread *t)
 {
   struct igc *gc = t->gc;
+  pthread_condattr_t condattr;
+  pthread_condattr_init (&condattr);
+  pthread_cond_init (&gc->cond, &condattr);
   mps_res_t res;
-  res = mps_ap_create_k (&t->dflt_ap, gc->dflt_pool, mps_args_none);
+  res = emacs_ap_create_k (&t->dflt_ap, gc->dflt_pool, mps_args_none);
   IGC_CHECK_RES (res);
-  res = mps_ap_create_k (&t->leaf_ap, gc->leaf_pool, mps_args_none);
+  res = emacs_ap_create_k (&t->leaf_ap, gc->leaf_pool, mps_args_none);
   IGC_CHECK_RES (res);
-  res = mps_ap_create_k (&t->immovable_ap, gc->immovable_pool, mps_args_none);
+  res = emacs_ap_create_k (&t->immovable_ap, gc->immovable_pool, mps_args_none);
   IGC_CHECK_RES (res);
   res = create_weak_ap (&t->weak_strong_ap, t, false);
   res = create_weak_hash_ap (&t->weak_hash_strong_ap, t, false);
@@ -3007,13 +3110,13 @@ igc_thread_remove (void **pinfo)
   destroy_root (&t->d.stack_root);
   destroy_root (&t->d.specpdl_root);
   destroy_root (&t->d.bc_root);
-  mps_ap_destroy (t->d.dflt_ap);
-  mps_ap_destroy (t->d.leaf_ap);
-  mps_ap_destroy (t->d.weak_strong_ap);
-  mps_ap_destroy (t->d.weak_weak_ap);
-  mps_ap_destroy (t->d.weak_hash_strong_ap);
-  mps_ap_destroy (t->d.weak_hash_weak_ap);
-  mps_ap_destroy (t->d.immovable_ap);
+  emacs_ap_destroy (&t->d.dflt_ap);
+  emacs_ap_destroy (&t->d.leaf_ap);
+  emacs_ap_destroy (&t->d.weak_strong_ap);
+  emacs_ap_destroy (&t->d.weak_weak_ap);
+  emacs_ap_destroy (&t->d.weak_hash_strong_ap);
+  emacs_ap_destroy (&t->d.weak_hash_weak_ap);
+  emacs_ap_destroy (&t->d.immovable_ap);
   mps_thread_dereg (deregister_thread (t));
 }
 
@@ -3677,7 +3780,7 @@ igc_on_idle (void)
   }
 }
 
-static mps_ap_t
+static emacs_ap_t *
 thread_ap (enum igc_obj_type type)
 {
   struct igc_thread_list *t = current_thread->gc_info;
@@ -3698,13 +3801,13 @@ thread_ap (enum igc_obj_type type)
       emacs_abort ();
 
     case IGC_OBJ_MARKER_VECTOR:
-      return t->d.weak_weak_ap;
+      return &t->d.weak_weak_ap;
 
     case IGC_OBJ_WEAK_HASH_TABLE_WEAK_PART:
-      return t->d.weak_hash_weak_ap;
+      return &t->d.weak_hash_weak_ap;
 
     case IGC_OBJ_WEAK_HASH_TABLE_STRONG_PART:
-      return t->d.weak_hash_strong_ap;
+      return &t->d.weak_hash_strong_ap;
 
     case IGC_OBJ_VECTOR:
     case IGC_OBJ_CONS:
@@ -3719,12 +3822,12 @@ thread_ap (enum igc_obj_type type)
     case IGC_OBJ_FACE_CACHE:
     case IGC_OBJ_BLV:
     case IGC_OBJ_HANDLER:
-      return t->d.dflt_ap;
+      return &t->d.dflt_ap;
 
     case IGC_OBJ_STRING_DATA:
     case IGC_OBJ_FLOAT:
     case IGC_OBJ_BYTES:
-      return t->d.leaf_ap;
+      return &t->d.leaf_ap;
     }
   emacs_abort ();
 }
@@ -3796,7 +3899,7 @@ igc_hash (Lisp_Object key)
    object.  */
 
 static mps_addr_t
-alloc_impl (size_t size, enum igc_obj_type type, mps_ap_t ap)
+alloc_impl_raw (size_t size, enum igc_obj_type type, mps_ap_t ap)
 {
   mps_addr_t p UNINIT;
   size = alloc_size (size);
@@ -3845,7 +3948,7 @@ alloc (size_t size, enum igc_obj_type type)
 alloc_immovable (size_t size, enum igc_obj_type type)
 {
   struct igc_thread_list *t = current_thread->gc_info;
-  return alloc_impl (size, type, t->d.immovable_ap);
+  return alloc_impl (size, type, &t->d.immovable_ap);
 }
 
 #ifdef HAVE_MODULES
@@ -4883,17 +4986,17 @@ igc_on_pdump_loaded (void *dump_base, void *hot_start, void *hot_end,
 igc_alloc_dump (size_t nbytes)
 {
   igc_assert (global_igc->park_count > 0);
-  mps_ap_t ap = thread_ap (IGC_OBJ_CONS);
+  emacs_ap_t *ap = thread_ap (IGC_OBJ_CONS);
   size_t block_size = igc_header_size () + nbytes;
   mps_addr_t block;
   do
     {
-      mps_res_t res = mps_reserve (&block, ap, block_size);
+      mps_res_t res = mps_reserve (&block, ap->mps_ap, block_size);
       if (res != MPS_RES_OK)
 	memory_full (0);
       set_header (block, IGC_OBJ_INVALID, block_size, 0);
     }
-  while (!mps_commit (ap, block, block_size));
+  while (!mps_commit (ap->mps_ap, block, block_size));
   return (char *) block + igc_header_size ();
 }
 

Warnings:

This is the "slow path" only, used for all allocations. Will cause a
great number of busy-looping threads.  Will be very slow.  Creating
additional emacs threads will result in a proportional number of
additional threads, which will be very, very slow, so don't.  Requires
pthread.h and stdatomic.h, and still does things not covered by those
APIs (memcpying over an atomic_uintptr_t, even if we know that its value
won't change, is probably verboten, and definitely should be).  I
*think* this code might work if we allocate from signal handlers, and I
think this code might work on systems that don't have lock-free atomics
(once the #error is removed), but it definitely won't do both at the
same time.

Pip

Re: Some experience with the igc branch

[Eli Zaretskii]

> Date: Mon, 23 Dec 2024 16:03:53 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>, ofv@wanadoo.es, emacs-devel@gnu.org, eller.helmut@gmail.com, acorallo@gnu.org
> 
> --- a/src/igc.c
> +++ b/src/igc.c
> @@ -747,19 +747,41 @@ IGC_DEFINE_LIST (igc_root);
>  
>  /* Registry entry for an MPS thread mps_thr_t.  */
>  
> +#include <pthread.h>

We cannot use pthreads.h in portable code.  If we want to use threads,
we need separate implementations for Posix and Windows, like wedid in
systhread.c for Lisp threads.

> +struct emacs_ap
> +{
> +  mps_ap_t mps_ap;
> +  struct igc *gc;
> +  pthread_t allocation_thread;

pthread_t is non-portable, for the same reasons.

> This is the "slow path" only, used for all allocations. Will cause a
> great number of busy-looping threads.

A lot of threads might be problematic.  Each thread reserves memory
for its stack, so you end up with lots of reserved memory, and on
32-bit systems can run out of address space.

Why do we need this, again?

Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Pip Cet <pipcet@protonmail.com> writes:
>>
>>> I'll spare you most of the details for now, but having read the mps
>>> header, MPS allocation is not safe to use from separate threads without
>>> locking the AP (or having per-thread APs), which we might end up doing
>>> on Windows, IIRC.
>>
>> Now I'm confused. We're using thread allocation points. See
>> create_thread_aps, thread_ap, and so on.
>
> I was confused.  This is only a problem if we allocate memory from a
> signal handler, which is effectively sharing the per-thread structure.
>
> (I'm still confused. My patch worked on the first attempt, which my code
> never does.  I suspect that while I made a mistake, it caused a subtle
> bug rather than an obvious one.)
>
> And we don't want to allocate memory from signal handlers, right?  We
> could, now (see warnings below):

Can't speak for others, but I wouldn't want it :-).

I can't cite myself, but I'm pretty sure I said some time ago already
that in a portable program one cannot do much in a signal handler in the
first place. So I wouldn't be surprised if MPS didn't support being
called from a signal handler. Not unreasonable for me.

But whatever. Maybe Richard Brooksby answers, and can shed light on that
or has ideas, if we don't overload him :-). And anyway, there is now
something workable in the igc branch. Maybe we could wait a bit, and
just proceed with something else meanwhile.

[... Thanks for the patch ...]

> Warnings:
>
> This is the "slow path" only, used for all allocations. Will cause a
> great number of busy-looping threads.  

Don't know why, but the busy looping threads makes me feel a bit
uncomfortable :-),

> Will be very slow. Creating additional emacs threads will result in a
> proportional number of additional threads, which will be very, very
> slow, so don't. Requires pthread.h and stdatomic.h, and still does
> things not covered by those APIs (memcpying over an atomic_uintptr_t,
> even if we know that its value won't change, is probably verboten, and
> definitely should be). I *think* this code might work if we allocate
> from signal handlers, and I think this code might work on systems that
> don't have lock-free atomics (once the #error is removed), but it
> definitely won't do both at the same time.
>
> Pip

BTW, do you know which signal handlers use Lisp, i.e. allocate Lisp
objects or access some? All? Or, would it be realistic to rewrite signal
handlers to not do that?

One thing I've seen done elsewhere is to publish a message to a message
board so that it can be handled outside of the signal handler. Something
like that, you know what I mean.