I this case, all the connections using up maximum work_mem is the potential threat. As said by Zoltan, work_mem is very high and shared_buffers as well.
Other considerations would be as following -
- Allocated kernel memory settings (like shmmax and shmget etc..)
- How much memory is used up by the system level processes (like root and non-pg users)
- It also depends on the database size and the amount of data being accessed across CPUs and memory.
- We need to ensure if unnecessary data is being read into the memory
( queries hitting non-vacuumed tables, slow performing queries, unnecessary full table scans etc)
Regards,
Venkat
On Tue, Aug 30, 2011 at 3:30 PM, Boszormenyi Zoltan <zb@xxxxxxxxxxx> wrote:
Hi,
2011-08-29 22:36 keltezéssel, Lonni J Friedman írta:
> ... I read that
> (max_connections * work_mem) should never exceed physical RAM, and ifwork_mem is process-local memory so
> that's accurate, then I suspect that's the root of my problem on
> systemA (below).
(max_connections * work_mem) < (physical RAM - shared_buffers)
Some queries may allocate multiples of work_mem, too.
Also, the kernel uses some memory for internal accounting, caching
and you need to account for the process binary in memory.
RAM (56GB) - shared_buffers (13GB) = 43GB
> However, I'd like confirmation before I start
> tweaking things, as one of these servers is in production, and I can't
> easily tweak settings to experiment (plus this problem takes a few
> weeks before swapping gets bad enough to impact performance).
>
> A few examples:
>
> 0) system A: 56GB RAM, running postgresql-8.4.8 with the following parameters:
> maintenance_work_mem = 96MB
> effective_cache_size = 40GB
> work_mem = 256MB
> wal_buffers = 16MB
> shared_buffers = 13GB
> max_connections = 300
which is less than
work_mem * max_connections = 300 * 0.25GB = 75GB
The system would start swapping before 43GB/0.25GB = 172 clients.
Similarly:
> 1) system B: 120GB RAM, running postgresql-9.0.4 with the following parameters:
> maintenance_work_mem = 1GB
> effective_cache_size = 88GB
> work_mem = 576MB
> wal_buffers = 4MB
> shared_buffers = 28GB
> max_connections = 200
120GB - 28GB = 92GB
is less than
work_mem * max_connections = 200 * 576MB = 112.5GB
Also, if you run anything else on the machine then the system would start
swapping much sooner than hitting max_connections number of clients.
I would never set work_mem that high by default. 8 - 16MB is usually
enough for the common case and you can set work_mem for special
queries from the client and then reset it.
Best regards,
Zoltán Böszörményi
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Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt, Austria
Web: http://www.postgresql-support.de
http://www.postgresql.at/
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