On Tue, Aug 30, 2011 at 3:00 AM, 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 if >> that's accurate, then I suspect that's the root of my problem on >> systemA (below). > > work_mem is process-local memory so > > (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. > >> 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 > > RAM (56GB) - shared_buffers (13GB) = 43GB > > which is less than > > work_mem * max_connections = 300 * 0.25GB = 75GB > > The system would start swapping before 43GB/0.25GB = 172 clients. > >> 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 > > Similarly: > > 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. Thanks for your reply. I've reduced shared_buffers to 8GB everywhere, which should definitely help. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
