Re: Make Signal handling patch platform-dependent?

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

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!