Re: Figuring out shared buffer pressure

On Wed, May 30, 2012 at 10:57 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Wed, May 30, 2012 at 10:38:10AM -0700, Jeff Janes wrote:
>> On Wed, May 30, 2012 at 9:56 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>> > As part of a blog, I started looking at how a user could measure the
>> > pressure on shared buffers, e.g. how much are they being used, recycled,
>> > etc.
>> >
>> > They way you normally do it on older operating systems is to see how
>> > many buffers on the free list (about to be reused) are reclaimed as
>> > needed --- that usually indicates kernel cache pressure.  Unfortunately,
>> > we don't have a freelist, except for initial assignment of shared
>> > buffers on startup.
>>
>> Isn't that what the buffers_alloc from pg_stat_bgwriter is ?
>
> The issue is that once a buffer is removed from the free list, it is
> never returned to the free list.

A buffer doesn't need to be removed from the linked list in order for
buffers_alloc to get incremented.

Conceptually, the freelist consists not only of the linked list, but
also of all unpinned buffers with a usagecount of zero.

Cheers,

Jeff