Re: Shared memory usage in PostgreSQL 9.1

Christoph Zwerschke <cito(at)online(dot)de> writes:
> ... This is a discrepancy of 1.25%.

> The difference could be explained by taking credit for the descriptors
> which may not be comprised in the shared_buffers setting, even if the
> shared_buffers value is set in memory units. But according to the docs,
> the descriptors should use 208/8192 = 2.5%. And on a 64bit machine, the
> descriptors should use even more memory, i.e. up to 5%.

> So I'm still a bit confused.

The long and the short of it is those numbers aren't meant to be exact.
If they were, we'd have to complicate the table to distinguish 32 vs 64
bit and possibly other factors, and we'd have to remember to re-measure
the values after any code change, neither of which seems worth the
trouble. Please note that the table itself says that (a) the values are
approximate, and (b) nobody has bothered to update the numbers since
8.3. Personally, I'm thrilled if you're seeing a discrepancy of only
1.25%.

regards, tom lane