Proof-of-concept ARC removal patches

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-patches(at)postgreSQL(dot)org
Subject: Proof-of-concept ARC removal patches
Date: 2005-02-04 02:41:52
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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.

To show how closely the code is related, I kept the ARC names of T1, T2,
and B1 for the corresponding 2Q lists (A1, Am, A1out respectively).
We'd probably rename to use the 2Q names if we apply this, but the
patches are smaller and easier to review as they are.

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.

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.

These should apply against CVS tip of either HEAD or REL8_0_STABLE
branch; note that "CVS tip" includes the bufmgr refactoring patch
I applied earlier today.


regards, tom lane

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