Wednesday, April 22, 2009

MySQL 5.4 Patches: Improvements to spin-loop

In InnoDB there is an implementation of both mutexes
and RW-locks. The RW-locks implementation have been
improved by the Google SMP patches. Both of these
implementation relies on spin-loops as part of their
implementation. The defaults in InnoDB is to check
the condition, if it's not ok to enter to spin for
about 5 microseconds and then come back to check the
condition again.

If one reads the Intel manual how to do spin-loops
they propose to use a PAUSE instruction and then
check the condition again, so a much more active
checking of the condition. When we tried this out
using the sysbench benchmark we found that using
the Intel approach worsened performance. So instead
we tried an approach of putting the PAUSE instruction
into the InnoDB spinloop instead.

This approach turned out to be a success. Even on
machines with only one thread per core we were able
to get a 3-4% increase in throughput. We also tried
various settings of the defaults of the time of
spinning in the spinloop and found that the original
default values were very close to the optimum values.
We found the optimum about 20% from the old default
values and made this slight change to the default
values of the spinloop.

It's my expectation that as we remove locks and the
mutexes and RW-locks gets less contended and there
are more locks where the threads are waiting that
this optimum value will change. The current best
setting is very likely to be governed by the fact
that the most waiting happens on very hot locks.
So with improvements of the mutexes and RW-locks
we should expect to see better performance with
a shorter time in the spinloop.

On the new SPARC CPU's that Sun has developed, the
CMT boxes, we used the results from the paper:
which stated that the optimum instruction to use
is a cache miss instruction, however as I don't
know how to program a cache miss instruction we
opted for the second best instruction which was a
dummy test-and-set instruction. So the PAUSE
instruction is replaced by a test-and-set instruction

We expect that the improvements due to this small
change is even bigger when there are multiple
threads per core since the contention on the
CPU pipeline is higher in those cases and it is
important that the spinloop stays away as much
as possible from being active executing

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