From: Helmut Eller <eller.helmut@gmail.com>
To: Pip Cet <pipcet@protonmail.com>
Cc: "Gerd Möllmann" <gerd@gnu.org>,
"Stefan Monnier" <monnier@iro.umontreal.ca>,
"Ihor Radchenko" <yantar92@posteo.net>,
emacs-devel@gnu.org
Subject: Re: MPS: marker-vector
Date: Tue, 06 Aug 2024 08:28:29 +0200 [thread overview]
Message-ID: <87v80e2mia.fsf@gmail.com> (raw)
In-Reply-To: <87bk26vfj1.fsf@protonmail.com> (Pip Cet's message of "Mon, 05 Aug 2024 21:14:13 +0000")
On Mon, Aug 05 2024, Pip Cet wrote:
>> This replaces the vector-with-free-list by a growable vector, i.e. the
>> free entries are always kept at the end of the vector.
>
> I don't think that's entirely accurate; it's quite possible for an entry
> at the beginning of the array to be splatted and remain untouched until
> the next time a DO_MARKERS reaches it, which may take a long time.
Indeed, that's true.
> I see one somewhat theoretical problem with this patch: if a marker is
> simultaneously kept in a weak hash table, it's possible for it to be
> splatted from the marker vector while remaining in the weak hash table
> (there's no guarantee all references will be splatted at the same time);
> if it is then retrieved from the weak hash table and made to point
> nowhere, we will try to remove it from the marker vector, and hit the
> igc_assert.
Never thought about this. Hm. I think MPS could easily guarantee that
all references are splatted at the same time: we could think of
splatting a reference like moving an object to address zero. MPS
certainly guarantees that all references to a moved object are updated
before they are seen by user code. MPS could do the same for splatted
references.
> The rest of my comments are tiny nits, really:
>
> - capacity isn't redundant on 32-bit systems
Not sure what you mean. Certainly igc_header_nwords works on 32-bit
systems too; and igc_header_nwords is pretty much the same as capacity.
> - I'd prefer the marker index to be signed; if it is unsigned, we
> don't need to assert it's >= 0, and assigning -1 to it confused me...
You'd have to talk to the person who designed the struct Lisp_Marker.
> - you shouldn't compare Lisp_Objects with ==
> - I'd prefer checking for splatted elements before deciding to grow
> the vector, if we can do so efficiently
Good idea.
> - I find XFIXNAT easier to read when the number is guaranteed to be nonnegative
> - using alloca is problematic for large vectors (which shouldn't be
> dumped, thus a nit)
Probably.
> I wonder whether it wouldn't be faster, upon encountering a marker that
> has been splatted, to fix the entire array all at once. That would
> ensure that creation order is respected, and splatting is relatively
> rare (and, when splatted, we can expect most of the array to have been
> splatted; indeed, I suspect it'd be best to give up on the marker vector
> and build a new one so the old one can be collected and we don't have to
> worry about never shrinking it).
Possibly. But I also decided to ignore the issue and happily let
somebody else come up with a benchmark that shows that any of this
matters.
next prev parent reply other threads:[~2024-08-06 6:28 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-07-04 4:59 Markers in a gap array Stefan Monnier
2024-07-04 10:24 ` Ihor Radchenko
2024-07-04 13:16 ` Stefan Monnier
2024-07-04 14:30 ` Ihor Radchenko
2024-07-04 20:11 ` Stefan Monnier
2024-07-04 20:34 ` Pip Cet
2024-07-04 20:42 ` Stefan Monnier
2024-07-17 16:48 ` Helmut Eller
2024-07-18 20:46 ` Stefan Monnier
2024-07-26 19:48 ` Helmut Eller
2024-08-05 19:54 ` MPS: marker-vector (was: Markers in a gap array) Helmut Eller
2024-08-05 21:14 ` MPS: marker-vector Pip Cet
2024-08-06 6:28 ` Helmut Eller [this message]
2024-08-06 6:51 ` Gerd Möllmann
2024-08-06 14:36 ` Pip Cet
2024-08-06 16:15 ` Helmut Eller
2024-08-06 3:59 ` Gerd Möllmann
2024-08-06 6:02 ` Helmut Eller
2024-07-04 22:24 ` Markers in a gap array Stefan Monnier
2024-07-07 12:31 ` Ihor Radchenko
2024-07-07 13:09 ` Konstantin Kharlamov
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