public inbox for [email protected]  

help / color / mirror / Atom feed

From: Alexandru Lazarev <[email protected]>
To: [email protected]
Subject: work_mem RAM Accounting in PostgreSQL
Date: Wed, 13 Nov 2024 21:09:35 +0200
Message-ID: <CAL93h0GtZ5H+X+8CRDOkcN1w0Bha2w3-57CqKSNFC2xrTNhEhA@mail.gmail.com> (raw)

Hello Everyone,

I have some questions regarding how the "work_mem" parameter affects the
overall RAM usage of PostgreSQL processes within a physical host or
container.

Each backend process during SQL execution may allocate N * "work_mem"
simultaneously. For example, if "work_mem" is set to 32MB and N=5 (i.e. 5
simultaneous and/or sequential ORDER and hash operations), and the initial
RAM usage (RSS - Resident Set Size) of the backend is 10MB, I would expect
the backend process to use 160MB (32MB * 5) + 10MB, resulting in a total
RAM usage of 170MB.

My questions are as follows:

1. What happens to the allocated "work_mem" after the execution of query
nodes? Are these memory allocations freed?
2. If they are freed, do they remain in the RSS of the PostgreSQL backend?
3. From various sources, I understand that these allocations are freed
after each node execution due to memory contexts, but they might remain in
some sort of backend memory pool for future reuse. Is this correct?
4. If so, will this memory be accounted for as used RAM on my
Linux/Container system after the backend returns to an idle state (e.g.,
connection pooling)?

Additionally: If the above is true, and my PostgreSQL host or container is
limited to 16GB of RAM, what would happen if I have 100 pooled connections,
each gradually allocating those 160MB? Will this memory be reclaimed (if I
understood it correctly as a kind of inactive anon mem), or will the OOM
Killer be triggered at some point (because it is real allocated memory)?

Thank you for your insights.

Best regards,
AlexL
Java Dev

view thread (3+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected]
  Subject: Re: work_mem RAM Accounting in PostgreSQL
  In-Reply-To: <CAL93h0GtZ5H+X+8CRDOkcN1w0Bha2w3-57CqKSNFC2xrTNhEhA@mail.gmail.com>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox