Re: Shared memory usage in PostgreSQL 9.1

Am 03.12.2011 18:02, schrieb Christoph Zwerschke:
> 400 MB = 419430400 Bytes
>
> but according to your log the used memory is:
>
> buffers = 424669472 Bytes
>
> 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%.

Just to clear up that last unexplained discrepancy, the problem is that
I wrongly assumed the descriptors were the only overhead to the shared
buffers. In reality it is more complex, e.g. additional memory for locks
is reserved for each shared buffer. The 208 Bytes in the docs refer to
the total overhead a shared buffer creates, while the value in Tomas'
log contained only the overhead caused by the descriptors.

Sorry for creating the noise and confusion.

-- Christoph