Re: work_mem RAM Accounting in PostgreSQL

On Mon, 2024-11-18 at 18:11 +0200, Alexandru Lazarev wrote:
> Highly appreciate your response. But I have some additional questions (inline)
>
> On Thu, Nov 14, 2024 at 4:40 PM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote:
> > On Wed, 2024-11-13 at 21:09 +0200, Alexandru Lazarev wrote:
> >
> > > My questions are as follows:
> > >
> > > 1. What happens to the allocated "work_mem" after the execution of query nodes? Are
> > >     these memory allocations freed?
> >
> > Yes.
> >
> > > 2. If they are freed, do they remain in the RSS of the PostgreSQL backend?
> >
> > They may, because the C library can choose not to actually free all the memory,
> > but retain some to serve future malloc() requests more efficiently.
>
> This part is important for me to understand (my apologies, I am not a C programmer
> and for me it is difficult reading PG sources :)): I wanted to understand if in this part
> there isn't some kind of allocated memory pooling in postgres. So, since some memory is
> freed then it is up to C Library and underlying OS if it will be returned back to OS or
> will stay somehow reserved - did I get it correctly? If so, then most probably this
> "reserved" memory should be reclaimed under memory pressure conditions (other backends
> processes try greedely allocate big chunks of memory)?

Idle PostgreSQL backends won't keep moch memory allocated under normal conditions:
the catalog cache, prepared statements, private memory to buffer temporary tables,
potentially a materialized WITH HOLD cursor.

Yes, it is up to the C library to keep some free'd memory allocated for reuse.
This memory won't be reclaimed when there is memory pressure, since it belongs to
the process.

> >
> >
> > The backends won't keep that much memory allocated, so you need not worry.
>
> Let me reformulate the use-case a bit differently - I would highly appreciate any community inputs: 
> 1) Let say 60 connections did some intensive memory consuming operations, each one
> allocating up to 200MB of work_mem, then they finished and returned to 'idle' state.

The lion's share of that memory will be freed when processing is done.

> 2) After that the rest of 40 connections starting doing "work_mem" consuming operations
> in parallel, each one allocating up to 300MB, then will N*"work_mem" allocated RAM
> from step#1 be reclaimed at point of step#2 when multiple backend need to allocate
> aggressively K*"wor_mem" memory?

Most of the memory is already reclaimed when the first cconnections are done, so you
shouldn't be in trouble.
All this is somehow vague, since it depends on the C library implementation.

I recommend that you start tuning with a value for work_mem that is low enough
to be very certain that you won't go out of memory.
Then monitor temporary files. If too many get created, increase work_mem, all the
while watching your memory consumption to avoid going OOM.
As soon as only few temporary files are created, you are probably good.
If you cannot get there, try to tune your queries.

The king's way to avoid problems in this area is to use a moderately sized
connection pool. Then connections don't sit idle for long and cannot hog much
memory.

Yours,
Laurenz Albe