Tuesday, May 19, 2009

Analysis of split flush list from buffer pool

In the Google v3 patch the buffer pool mutex have been
split into an array of buffer page hash mutexes and a
buffer flush list mutex and the buffer pool mutex also

I derived the patch splitting out the buffer flush list
mutex from the Google v3 patch against the MySQL 5.4.0
tree. The patch is here.

I derived a lot of prototype patches based on MySQL 5.4.0
and Dimitri tried them out. This particular patch seems
to be the most successful in the pack of patches we
tested. It had a consistent positive impact.

The main contribution of this patch is twofold. It
decreases the pressure on the buffer pool mutex by
splitting out a critical part where the oldest dirty
pages are flushed out to disk. In addition this patch
also decreases the pressure on the log_sys mutex by
releasing the log_sys mutex earlier for the mini-
transactions. In addition it removes interaction
between the buffer pool mutex and the log_sys mutex.
So previously both mutexes had to be held for a
while, this is no longer necessary since only the
flush list mutex is needed, not the buffer pool

The new patch is the b11 variant which is red in
the comparison graphs.

As we can see the read-write tests have a pretty significant boost
from this patch, it improves top performance by 5% and by 10-20%
on higher number of threads. It also moves the maximum from 16 to
32 threads.

Even on read-only there are some positive improvements although
it is very possible those are more random in nature.

Finally the above picture shows that this patch also moves the
optimal InnoDB thread concurrency up to 24 from 16 since it
allows for more concurrency inside InnoDB. This is also visible
by looking at the numbers for InnoDB Thread Concurrency set to 0
as seen below.

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