Re: Some experience with the igc branch

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

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

>> 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.

Noted.

As an aside, without any relevance to the fact that we should avoid
using them, aren't pthreads available on "mingw"64 systems?

>> +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.

This is a PoC.  While we shouldn't share structures between Emacs-side
threads, we should of course use (at most) a single allocation thread
rather than one per thread per AP.  Also, yield the CPU once in a while
:-)

> Why do we need this, again?

We can't interrupt allocation, so we move it to a separate thread where
it will complete (unlocking the arena) even if a signal interrupts us.

Pip