Re: PostgreSQL processes use large amount of private memory on Windows

On Thu, Sep 17, 2020 at 7:27 PM Peter J. Holzer <hjp-pgsql(at)hjp(dot)at> wrote:

> What I would do:
>
> * Set log_statement to all (warning: that can be a lot of log messages.
> It can also be a privacy/security hazard, depending on who has access
> to the server and how sensitive queries are).
> * Frequently (at least once per minute) record the size of all postgres
> processes. Send an alert if one of them is "too large".
>
> This should give you a good idea what the processes were doing at the
> time they allocated that memory, so that you can reproduce the problem.
>

In fact I already did track the memory behavior over time and recorded
memory utilization every 5 minutes. The memory consumption exhibits a
surprisingly linear growth of about 400 MB per hour. And since I can't see
any sharp points where the memory jumps I can't really identify any single
query culprit that is causing this. It seems more like there are a large
number of small queries that are causing the memory to stack up over time.

And again, the real mystery here is that we don't see this behavior in most
deployments of the product which all utilize the database in the same way.
This leads me to think that there is some configuration on the server side
that is somehow affecting memory allocation. It's almost as if the shared
memory allocation between processes is not working and that all processes
are holding a copy of the shared memory instead of actually sharing.

--
Best regards, Øystein Kolsrud

On Thu, Sep 17, 2020 at 7:27 PM Peter J. Holzer <hjp-pgsql(at)hjp(dot)at> wrote:

> On 2020-09-17 11:19:07 +0200, Øystein Kolsrud wrote:
> > I have a product that uses PostgreSQL (v9.6) as database on Windows, and
> we
> > often observe that the "shared working set" memory reported for the
> individual
> > connection processes is quite high (around 200MB), but the "private
> working
> > set" is relatively low (around 5 MB). I assume this is due to the
> processes
> > utilizing the shared buffers of Postgres, and in these cases everything
> works
> > fine as the total memory consumption on the system is quite low. But for
> some
> > deployments we have observed that the "private working set" is around
> 200 MB as
> > well, and in these cases the server quickly runs out of memory when the
> number
> > of connections rises. I have never been able to reproduce this behavior
> myself.
> [...]
> > All settings typically referred to (like "work_mem" and "temp_buffers")
> are at
> > default settings, and I have a very hard time seeing how those settings
> could
> > add up to memory in the magnitude of 200MB.
>
> The work_mem setting is per operation (e.g. sort, hash join, etc.), so a
> complex query may use a multiple of it. However, the default is just 4 MB,
> so a query would have to be very complex to use 50 times as much. Also,
> that memory is freed after the query (however "freed" does not
> necessarily mean "returned to the OS", it can just mean "marked as
> availiable for reuse" - that depends on the C library and the OS).
>
> What I would do:
>
> * Set log_statement to all (warning: that can be a lot of log messages.
> It can also be a privacy/security hazard, depending on who has access
> to the server and how sensitive queries are).
> * Frequently (at least once per minute) record the size of all postgres
> processes. Send an alert if one of them is "too large".
>
> This should give you a good idea what the processes were doing at the
> time they allocated that memory, so that you can reproduce the problem.
>
> hp
>
> --
> _ | Peter J. Holzer | Story must make more sense than reality.
> |_|_) | |
> | | | hjp(at)hjp(dot)at | -- Charles Stross, "Creative writing
> __/ | http://www.hjp.at/ | challenge!"
>

--
Mvh Øystein Kolsrud