Re: Add the ability to limit the amount of memory that can be allocated to backends.

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Andrei Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
Cc: reid(dot)thompson(at)crunchydata(dot)com, Arne Roland <A(dot)Roland(at)index(dot)de>, Andres Freund <andres(at)anarazel(dot)de>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, vignesh C <vignesh21(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, "stephen(dot)frost" <stephen(dot)frost(at)crunchydata(dot)com>
Subject: Re: Add the ability to limit the amount of memory that can be allocated to backends.
Date: 2023-10-18 19:00:42
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* Andrei Lepikhov (a(dot)lepikhov(at)postgrespro(dot)ru) wrote:
> On 29/9/2023 09:52, Andrei Lepikhov wrote:
> > On 22/5/2023 22:59, reid(dot)thompson(at)crunchydata(dot)com wrote:
> > > Attach patches updated to master.
> > > Pulled from patch 2 back to patch 1 a change that was also pertinent
> > > to patch 1.
> > +1 to the idea, have doubts on the implementation.
> >
> > I have a question. I see the feature triggers ERROR on the exceeding of
> > the memory limit. The superior PG_CATCH() section will handle the error.
> > As I see, many such sections use memory allocations. What if some
> > routine, like the CopyErrorData(), exceeds the limit, too? In this case,
> > we could repeat the error until the top PG_CATCH(). Is this correct
> > behaviour? Maybe to check in the exceeds_max_total_bkend_mem() for
> > recursion and allow error handlers to slightly exceed this hard limit?

> By the patch in attachment I try to show which sort of problems I'm worrying
> about. In some PП_CATCH() sections we do CopyErrorData (allocate some
> memory) before aborting the transaction. So, the allocation error can move
> us out of this section before aborting. We await for soft ERROR message but
> will face more hard consequences.

While it's an interesting idea to consider making exceptions to the
limit, and perhaps we'll do that (or have some kind of 'reserve' for
such cases), this isn't really any different than today, is it? We
might have a malloc() failure in the main path, end up in PG_CATCH() and
then try to do a CopyErrorData() and have another malloc() failure.

If we can rearrange the code to make this less likely to happen, by
doing a bit more work to free() resources used in the main path before
trying to do new allocations, then, sure, let's go ahead and do that,
but that's independent from this effort.



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