Re: Idle power fix regresses ebizzy performance (was 3.12-stable backport of NUMA balancing patches)

[Mel Gorman <mgorman@suse.de>]

On Wed, Jan 08, 2014 at 01:48:58PM +0000, Mel Gorman wrote:
> Adding LKML to the list as this -stable snifftest has identified an
> upstream regression.
> 

This is a false alarm.

The test machine in question was originally installed based on a beta
version of openSUSE 13.1. It included a package by default that set default
malloc parameters that I was not aware. Normally the package is there to
catch bugs during beta testing and removed before a GA release but it's
left in place if a user does a distribution update.

With the debugging RPM installed, the free paths contended on a global
mutex in glibc.  Ebizzy had been classified as a CPU intensive and memory
free intensive benchmark (not that common) but turbostat showed that the
CPUs were over 95% of the time in C6 and mpstat verified that the CPUs
were mostly idle. It did not take long to see that everything was blocked
waiting on a futex and to identify where it was in glibc. It's only a
factor when malloc debugging is enabled so normally people would not see it.

The "regression" is because CPUs are reaching C6 as they should and there
is a delay when exiting it. This is behaving as designed and fixing this
would involve doing something stupid. Once the problem RPM was removed
ebizzy performed as expected. 3.13-rc7, the revert and forcing max_cstate=1
all have similar performance.

Sorry about the noise.

-- 
Mel Gorman
SUSE Labs