Re: Clock sweep not caching enough B-Tree leaf pages?

On Wed, Apr 15, 2015 at 10:07 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Wed, Apr 15, 2015 at 12:15 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > IIUC, this will allow us to increase usage count only when the buffer
> > is touched by clocksweep to decrement the usage count.
>
> Yes.
>
> > I think such a solution will be good for the cases when many evictions
> > needs to be performed to satisfy the workload, OTOH when there are
> > not too many evictions that needs to be done, in such a case some of
> > the buffers that are accessed much more will have equal probability to
> > get evicted as compare to buffers which are less accessed.
>
> Possibly, but I think it's even worse under the current algorithm.
> Under this proposal, if we go for a long time without any buffer
> evictions, every buffer usage's count will top out at 2 more than
> wherever it was after the last clock sweep. In the worst case, every
> buffer (or most of them) could end up with the same usage count. But
> under the status quo, they'll all go to 5, which is an even bigger
> loss of information, and which will make the first eviction much more
> expensive than if they are all pegged at 2 or 3.
>

Okay, got your point. On further thinking on this idea, it occurred to me
that with this idea you are trying to reduce the clock-sweep time which
in-turn will inturn improve the chances of finding usable buffer in less
time under high pressure, if that is true then I think we should have seen
the similar gain even with the bgreclaimer idea which I have tried sometime
back, because that has also reduced the time for clock-sweep.

> There could be ways to improve things further, such as by slowly
> advancing the clock sweep even when no eviction is required. But
> that's a bit tricky to do, too. You'd like (perhaps) to advance it
> one step for every buffer allocation, but you don't want a centralized
> counter, or unnecessary contention on the clock sweep when no eviction
> is necessary in the first place. There's probably some way to make it
> work, though, if we put our mind to it. I'm inclined to try the
> simpler approach first and see what it buys.
>

Sure, I think it is worth a try.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com