> Tom Lane wrote
> Attached are two different variants of a patch to remove the ARC
> cache algorithm in favor of variants of the "2Q" algorithm.
>
> The first patch approximates the "full 2Q" algorithm described by
> Johnson and Shasha, while the second approximates their
> "simplified 2Q"
> method. Full 2Q uses a list of pages that were recently in cache but
> no longer are, while simplified 2Q does not; the first patch is
> therefore a smaller change to the existing code.
"Full 2Q" is the right one, I think.
> The patches are not exactly the J&S algorithms, in part because I kept
> the special cases for VACUUM, and in part because I really
> don't believe
> their idea about not promoting from T1 into T2 until the page
> has fallen
> into B1. This helped to minimize the change from the
> existing ARC code.
Agreed.
> In some desultory testing with pgbench, there was no significant
> performance difference among the three algorithms, which leads me to
> think that simplified 2Q might be the best bet (it certainly has the
> smallest memory footprint). But it would be a good idea to do some
> more measurements before believing that. I hope that Mark
> Wong can give
> these a try on his setup soon.
Will give these a whirl...
> Comments?
>
The use of 25% T1 and 75% T2 is probably the only thing to discuss, for
me.
Johnson & Sasha's results show that having a larger T1 makes the system
more responsive to changes, while a larger T2 gives a better longer term
hit rate. J&S's results show that higher T1/T2 ratios are better with
smaller caches. The CARS results show that keeping T1 above a certain
minimum size is better also.
From that, I'd suggest we choose a T1/T2 ratio that changes according to
how you start shared_buffers. If we had another GUC at all, it should be
one that isn't listed in the postgresql.conf file at all...since it
would be easy to misuse.
A default setting of something like T1 as % of total (just roughly)
= 50% when shared_buffers <= 1000
= 25% when shared_buffers = 5000
= 10% when shared_buffers = 20000
with smoothing...
Best Regards, Simon Riggs
