"McKinzie, Alan (Alan)" <[email protected]> writes:
> I am trying to understand how memory is allocated/used by our Postgresql
> Database connections. From reading the documentation it appears that
> work_mem and temp_buffers are the 2 largest contributors (and work_mem usage
> can grow) to the memory utilized by the Database connections. In addition,
> if I understand correctly, work_mem and temp_buffers each have their own
> pool, and thus database connections use (when needed) and return memory to
> these pools. I have not read this anywhere, but based on observation it
> appears that once these pools grow, they never release any of the memory
> (e.g. they do not shrink in size if some of the memory has not been for a
> given period of time).
Temp buffers, once used within a particular backend process, are kept
for the life of that process. Memory consumed for work_mem will be
released back to libc at the end of the query. The net effect of that
is platform-dependent --- my experience is that glibc on Linux is able
to give memory back to the OS, but on other platforms the process memory
size doesn't shrink.
> With that said, are there any mechanisms available to determine how much
> work_mem and temp_buffers memory has been allocated by the Postgres database
> (and by database connection/process would be very useful as well)? Also,
> which postgres process manages the memory pools for work_mem and temp_buffers?
There's no "pool", these allocations are process-local.
> FYI â I am using smem (on a linux server) to monitor the memory allocated
> to our Database connections. In an attempt to lower our memory footprint, I
> lowered our setting for work_mem from 1MB down to 500kB (in addition I
> enabled log_temp_files to see the SQL statements that now use temp files for
> sorting and hash operations). As I expected the memory used by the
> connections that were doing large sorts went down in size. However, one of
> those DB connections dramatically increased in memory usage with this change.
> It went from approx. 6MB up to 37MB in memory usage?
Keep in mind that work_mem is the max per sort or hash operation, so a
complex query could consume a multiple of that. The most obvious theory
about your result is that the work_mem change caused the planner to
switch to another plan that involved more sorts or hashes than before.
But without a lot more detail than this, we can only speculate.
> Are temp_buffers used in conjunction with some sorting operations that use
> temp_files (and thus this connection allocated several temp_buffers?
No, they're only used in connection with temp tables.
regards, tom lane
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