Re: [RFC][PATCH 00/13] percpu rwsem -v2

From: Daniel Wagner <daniel.wagner@bmw-carit.de>
To: Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>
Cc: <oleg@redhat.com>, <paulmck@linux.vnet.ibm.com>, <tj@kernel.org>,
	<mingo@redhat.com>, <linux-kernel@vger.kernel.org>,
	<der.herr@hofr.at>, <dave@stgolabs.net>, <riel@redhat.com>,
	<viro@ZenIV.linux.org.uk>, <torvalds@linux-foundation.org>,
	<jlayton@poochiereds.net>
Subject: Re: [RFC][PATCH 00/13] percpu rwsem -v2
Date: Wed, 24 Jun 2015 11:18:34 +0200	[thread overview]
Message-ID: <558A75EA.40905@bmw-carit.de> (raw)
In-Reply-To: <20150624084648.GB27873@gmail.com>

On 06/24/2015 10:46 AM, Ingo Molnar wrote:
> So I'd suggest to first compare preemption behavior: does the workload 
> context-switch heavily, and is it the exact same context switching rate and are 
> the points of preemption the same as well between the two kernels?

If I read this correctly, the answer is yes.

First the 'stable' flock02 test:

perf stat --repeat 5  --pre 'rm -rf /tmp/a' ~/src/lockperf/flock02 -n 128 -l 64 /tmp/a
0.008793148
0.008784990
0.008587804
0.008693641
0.008776946

 Performance counter stats for '/home/wagi/src/lockperf/flock02 -n 128 -l 64 /tmp/a' (5 runs):

         76.509634      task-clock (msec)         #    3.312 CPUs utilized            ( +-  0.67% )
                 2      context-switches          #    0.029 K/sec                    ( +- 26.50% )
               128      cpu-migrations            #    0.002 M/sec                    ( +-  0.31% )
             5,295      page-faults               #    0.069 M/sec                    ( +-  0.49% )
        89,944,154      cycles                    #    1.176 GHz                      ( +-  0.66% )
        58,670,259      stalled-cycles-frontend   #   65.23% frontend cycles idle     ( +-  0.88% )
                 0      stalled-cycles-backend    #    0.00% backend  cycles idle   
        76,991,414      instructions              #    0.86  insns per cycle        
                                                  #    0.76  stalled cycles per insn  ( +-  0.19% )
        15,239,720      branches                  #  199.187 M/sec                    ( +-  0.20% )
           103,418      branch-misses             #    0.68% of all branches          ( +-  6.68% )

       0.023102895 seconds time elapsed                                          ( +-  1.09% )

And here posix01 which shows high variance:

perf stat --repeat 5  --pre 'rm -rf /tmp/a' ~/src/lockperf/posix01 -n 128 -l 64 /tmp/a
0.006020402
32.510838421
55.516466069
46.794470223
5.097701438

 Performance counter stats for '/home/wagi/src/lockperf/posix01 -n 128 -l 64 /tmp/a' (5 runs):

       4177.932106      task-clock (msec)         #   14.162 CPUs utilized            ( +- 34.59% )
            70,646      context-switches          #    0.017 M/sec                    ( +- 31.56% )
            28,009      cpu-migrations            #    0.007 M/sec                    ( +- 33.55% )
             4,834      page-faults               #    0.001 M/sec                    ( +-  0.98% )
     7,291,160,968      cycles                    #    1.745 GHz                      ( +- 32.17% )
     5,216,204,262      stalled-cycles-frontend   #   71.54% frontend cycles idle     ( +- 32.13% )
                 0      stalled-cycles-backend    #    0.00% backend  cycles idle   
     1,901,289,780      instructions              #    0.26  insns per cycle        
                                                  #    2.74  stalled cycles per insn  ( +- 30.80% )
       440,415,914      branches                  #  105.415 M/sec                    ( +- 31.06% )
         1,347,021      branch-misses             #    0.31% of all branches          ( +- 29.17% )

       0.295016987 seconds time elapsed                                          ( +- 32.01% )

BTW, thanks for the perf stat tip. Really handy!

cheers,
daniel