Re: 2nd Level Buffer Cache

On Thu, Mar 24, 2011 at 12:36 PM, Jim Nasby <jim@nasby.net> wrote:
> On Mar 23, 2011, at 5:12 PM, Tom Lane wrote:
>> Robert Haas <robertmhaas@gmail.com> writes:
>>> It looks like the only way anything can ever get put on the free list
>>> right now is if a relation or database is dropped.  That doesn't seem
>>> too good.
>>
>> Why not?  AIUI the free list is only for buffers that are totally dead,
>> ie contain no info that's possibly of interest to anybody.  It is *not*
>> meant to substitute for running the clock sweep when you have to discard
>> a live buffer.
>
> Turns out we've had this discussion before: http://archives.postgresql.org/pgsql-hackers/2010-12/msg01088.php and
http://archives.postgresql.org/pgsql-hackers/2010-12/msg00689.php
>
> Tom made the point in the first one that it might be good to proactively move buffers to the freelist so that
backendswould normally just have to hit the freelist and not run the sweep. 
>
> Unfortunately I haven't yet been able to do any performance testing of any of this... perhaps someone else can try
andmeasure the amount of time spent by backends running the clock sweep with different shared buffer sizes. 

I tried under the circumstances I thought were mostly likely to show a
time difference, and I was unable to detect a reliable difference in
timing between free list and clock sweep.


Cheers,

Jeff