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

On Wed, Apr 16, 2014 at 1:44 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
>> Anyways, I'm still curious if you can post similar numbers basing the
>> throttling on gross allocation counts instead of time. Meaning: some
>> number of buffer allocations has to have occurred before you consider
>> eviction. Besides being faster I think it's a better implementation:
>> an intermittently loaded server will give more consistent behavior.
>
> Yeah --- I think wall-clock-based throttling is fundamentally the wrong
> thing anyway. Are we going to start needing a CPU speed measurement to
> tune the algorithm with? Not the place to be going. But driving it off
> the number of allocations that've been done could be sensible. (OTOH,
> that means you need a central counter, which itself would be a
> bottleneck.)

sure -- note we already track that in BufferStrategyControl
(everything in buffer allocation is already centrally managed
essentially).

/*
* Statistics. These counters should be wide enough that they can't
* overflow during a single bgwriter cycle.
*/
uint32 completePasses; /* Complete cycles of the clock sweep */
uint32 numBufferAllocs; /* Buffers allocated
since last reset */

merlin