Date: Mon, 13 Jan 2014 19:24:06 +0000
From: Mel Gorman <mgorman@suse.de>
To: Len Brown <len.brown@intel.com>
Cc: athorlton@sgi.com, riel@redhat.com, chegu_vinod@hp.com,
        Greg KH <gregkh@linuxfoundation.org>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org
Subject: Re: Idle power fix regresses ebizzy performance (was 3.12-stable
 backport of NUMA balancing patches)
Message-ID: <20140113192406.GL27046@suse.de>
References: <1389103248-17617-1-git-send-email-mgorman@suse.de>
 <20140107141715.GA32491@kroah.com>
 <20140107185440.GA7844@kroah.com>
 <20140107203012.GA27046@suse.de>
 <20140108104340.GC27046@suse.de>
 <20140108134858.GF27046@suse.de>
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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
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