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

> Perhaps this isn't the help you were looking for, but I spent a long time
> looking into this a few years ago.  Then I stopped and decided to work on
> other things.  I would recommend you do so too.

Agreed. It seems that my concerns were not valid, and since you have
already done some testing here, it further closes the matter.


> 4) If most, but not quite all, of the highly-used data fits shared_buffers
> and shared_buffers takes most of RAM (or at least, most of RAM not already
> needed for other things like work_mem and executables), then the replacement
> policy matters a lot.  But different policies suit different work-loads, and
> there is little reason to think we can come up with a way to choose between
> them.  (Also, in these conditions, performance is very chaotic.  You can run
> the same algorithm for a long time, and it can suddenly switch from good to
> bad or the other way around, and then stay in that new mode for a long
> time).  Also, even if you come up with a good algorithm, if you make the
> data set 20% smaller or 20% larger, it is no longer a good algorithm.

Does that mean that an ideal, high performance postgres setup should
*never* set the shared_buffers to a large percentage of the system's
RAM? If so, have we ever encountered issues with low specs systems?


> 5) Having buffers enter with usage_count=0 rather than 1 would probably be
> slightly better most of the time under conditions described in 4, but there
> is no way get enough evidence of this over enough conditions to justify
> making a change.  And besides, how often do people run with shared_buffers
> being most of RAM, and the hot data just barely not fitting in it?

Agreed.

> 1) If all data fits in RAM but not shared_buffers, and you have a very large
> number of CPUs and a read-only or read-mostly workload, then BufFreelistLock
> can be a major bottle neck.  (But, on a Amazon high-CPU instance, I did not
> see this very much.  I suspect the degree of problem depends a lot on
> whether you have a lot of sockets with a few CPUs each, versus one chip with
> many CPUs).  This is very easy to come up with model cases for, pgbench -S
> -c8 -j8, for example, can often show it.

I will try that, thanks.

> 2) A major reason that people run with shared_buffers much lower than RAM is
> that performance seems to suffer with shared_buffers > 8GB under write-heavy
> workloads, even with spread-out checkpoints.  This is frequently reported as
> a real world problem, but as far as I know has never been reduced to a
> simple reproducible test case. (Although there was a recent thread, maybe
> "High CPU usage / load average after upgrading to Ubuntu 12.04", that I
> thought might be relevant to this.  I haven't had the time to seriously
> study the thread, or the hardware to investigate it myself)
>

This seems interesting.Do we have some indications as to what the
problems could be?

Regards,

Atri



--
Regards,

Atri
l'apprenant