Re: Page replacement algorithm in buffer cache - Mailing list pgsql-hackers

On 4/1/13 4:55 PM, Merlin Moncure wrote:
> On Mon, Apr 1, 2013 at 4:09 PM, Andres Freund<andres@2ndquadrant.com>  wrote:
>> >On 2013-04-01 08:28:13 -0500, Merlin Moncure wrote:
>>> >>On Sun, Mar 31, 2013 at 1:27 PM, Jeff Janes<jeff.janes@gmail.com>  wrote:
>>>> >> >On Friday, March 22, 2013, Ants Aasma wrote:
>>>>> >> >>
>>>>> >> >>On Fri, Mar 22, 2013 at 10:22 PM, Merlin Moncure<mmoncure@gmail.com>
>>>>> >> >>wrote:
>>>>>> >> >> >well if you do a non-locking test first you could at least avoid some
>>>>>> >> >> >cases (and, if you get the answer wrong, so what?) by jumping to the
>>>>>> >> >> >next buffer immediately.  if the non locking test comes good, only
>>>>>> >> >> >then do you do a hardware TAS.
>>>>>> >> >> >
>>>>>> >> >> >you could in fact go further and dispense with all locking in front of
>>>>>> >> >> >usage_count, on the premise that it's only advisory and not a real
>>>>>> >> >> >refcount.  so you only then lock if/when it's time to select a
>>>>>> >> >> >candidate buffer, and only then when you did a non locking test first.
>>>>>> >> >> >  this would of course require some amusing adjustments to various
>>>>>> >> >> >logical checks (usage_count <= 0, heh).
>>>>> >> >>
>>>>> >> >>Moreover, if the buffer happens to miss a decrement due to a data
>>>>> >> >>race, there's a good chance that the buffer is heavily used and
>>>>> >> >>wouldn't need to be evicted soon anyway. (if you arrange it to be a
>>>>> >> >>read-test-inc/dec-store operation then you will never go out of
>>>>> >> >>bounds) However, clocksweep and usage_count maintenance is not what is
>>>>> >> >>causing contention because that workload is distributed. The issue is
>>>>> >> >>pinning and unpinning.
>>>> >> >
>>>> >> >
>>>> >> >That is one of multiple issues.  Contention on the BufFreelistLock is
>>>> >> >another one.  I agree that usage_count maintenance is unlikely to become a
>>>> >> >bottleneck unless one or both of those is fixed first (and maybe not even
>>>> >> >then)
>>> >>
>>> >>usage_count manipulation is not a bottleneck but that is irrelevant.
>>> >>It can be affected by other page contention which can lead to priority
>>> >>inversion.  I don't be believe there is any reasonable argument that
>>> >>sitting and spinning while holding the BufFreelistLock is a good idea.
>> >
>> >In my experience the mere fact of (unlockedly, but still) accessing all the
>> >buffer headers can cause noticeable slowdowns in write only/mostly workloads with
>> >big amounts of shmem.
>> >Due to the write only nature large amounts of the buffers have a similar
>> >usagecounts (since they are infrequently touched after the initial insertion)
>> >and there are no free ones around so the search for a buffer frequently runs
>> >through*all*  buffer headers multiple times till it decremented all usagecounts
>> >to 0. Then comes a period where free buffers are found easily (since all
>> >usagecounts from the current sweep point onwards are zero). After that it
>> >starts all over.
>> >I now have seen that scenario multiple times:(
> Interesting -- I was thinking about that too, but it's a separate
> problem with a different trigger.  Maybe a bailout should be in there
> so that after X usage_count adjustments the sweeper summarily does an
> eviction, or maybe the "max" declines from 5 once per hundred buffers
> inspected or some such.

What's the potential downside on that though? IE: what happens if this scheme suddenly evicts the root page on a
heavilyused index? You'll suddenly have a ton of stuff blocked waiting for that page to come back in.
 

This is a use case that I think would benefit greatly from a background process that keeps pages in the free list.

That said, I now suspect that your "frightened turtle" approach would be of higher value than "bgfreelist"... but I
suspectwe'll ultimately want both of them for different reasons.
 
--
Jim C. Nasby, Data Architect                   jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net