On 3.12.2011 13:39, Christoph Zwerschke wrote: > For a PostgreSQL 9.1.1 instance, > I have used the following postgresql.conf settings: > > max_connections = 100 > shared_buffers = 400MB > wal_buffers = 16MB > > All the other parameters have been left as default values. > > When I startup the instance, I get an error message > saying that the shared memory does not suffice > and 451837952 Bytes would be used. > > However, this is not what I expect when calculating > the needs according to the documentation, Table 17-2 at > http://www.postgresql.org/docs/current/static/kernel-resources.html > > According to that table the usage would be: > Connections: 1908000 Bytes > Autovac workers: 57240 Bytes > Prepared transactions: 0 Bytes > Shared disk buffers: 400MB > WAL buffers: 16MB > Fixed space: 788480 Bytes > Sum: 435145336 > > This is about 16MB less than what is really requested. > > How can this substantial discrepancy be explained? > > For PostgreSQL 9.1, some important item must be missing > in Table 17-2, or some values are wrong. Hi, the documentation is not exact, in this area. It's rather an overview than exhaustive description, but I admit it's a bit confusing. There are internal pieces that are not accounted for in the docs, and part of the discrepancy probably comes from 32bit vs. 64bit differences. Do you need to know an exact value or are you just interested why the values in docs are not exact? If you want to see what exactly needs how much memory, check the src/backend/storage/ipc/ipci.c file in sources. I've added some log messages, and this is the result on my 64bit machine (using the values you've posted): WARNING: initial = 100000 WARNING: hash estimate = 12368 WARNING: buffers = 424669472 WARNING: locks = 2509584 WARNING: predicate locks = 2278566 WARNING: proc global = 70237 WARNING: xlogs = 16803120 WARNING: clogs = 131360 WARNING: subtrans = 263040 WARNING: two-phase commits = 16 WARNING: multi-xacts = 198224 WARNING: lwlocks = 3282376 WARNING: proc array = 864 WARNING: backend status = 141440 WARNING: sinval = 67224 WARNING: pm signal = 872 WARNING: proc signal = 3960 WARNING: bgwriter = 1228840 WARNING: autovacuum = 216 WARNING: wal sender = 28 WARNING: wal receiver = 1072 WARNING: btree = 1260 WARNING: sync scan = 656 WARNING: async = 67112 WARNING: final size = 451985408 Tomas -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
