Re: Figuring out shared buffer pressure

On Wed, May 30, 2012 at 2:55 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Wed, May 30, 2012 at 11:51:23AM -0700, Jeff Janes wrote:
>> On Wed, May 30, 2012 at 11:23 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>> > On Wed, May 30, 2012 at 11:06:45AM -0700, Jeff Janes wrote:
>> >> 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:
>> >> >>
>> >> >> 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.
>> >
>> > Seems buffers_alloc is the number of calls to StrategyGetBuffer(), which
>> > tells how many time we have requested a buffer.  Not sure how that helps
>> > measure buffer pressure.
>>
>> Once the linked list is empty, every request for a buffer to read a
>> new page into must result in the eviction of the previous occupant
>> from this conceptual freelist buffer (except perhaps for some race
>> conditions).  Isn't that what you wanted?  Except that the
>> buffers_alloc does not get incremented when the StrategyGetBuffer is
>> satisfied by a ring strategy rather than the default strategy.
>
> Well, the ideal case is that I could find out how often data that is
> near to be discarded is actually needed, hence the "reclaimed" field
> that is often important for kernel memory presssure reporting on older
> operating systems.  I will post an email soon about my theory of why
> buffer pressure is an important thing to report to users.

Ah, now I see. By reclaimed I thought you meant claimed for reuse
with a new page, but you mean it was found to already have the page we
wanted and a usagecount of zero and was unpinned, and so would have
been in danger of eviction if we hadn't just now pinned it and bumped
the usagecount. Yeah, I don't think anything currently reported will
help with that.

Cheers,

Jeff