linux-kernel - Re: perf tools miscellaneous questions

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Date:	Thu, 04 Nov 2010 23:28:04 +0100
From:	Victor Jimenez <victor.javier@....es>
To:	Francis Moreau <francis.moro@...il.com>
CC:	Reid Kleckner <reid.kleckner@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	linux-perf-users@...r.kernel.org
Subject: Re: perf tools miscellaneous questions

On 11/04/2010 09:58 PM, Francis Moreau wrote:
> Francis Moreau<francis.moro@...il.com>  writes:
>
>> Francis Moreau<francis.moro@...il.com>  writes:
>>
> [...]
>
>>> How could I know the number of cache level on my cpu ?
>>>
>>> I tried:
>>>
>>>    $ perf stat -e cache-misses:u,l1d-loads-misses:u true
>>>
>>>      Performance counter stats for 'true':
>>>
>>>                     802  cache-misses
>>>                     937  L1-dcache-load-misses
>>>
>>>             0.000996578  seconds time elapsed
>>>
>>>    $ perf stat -e cache-misses:u,L2-loads-misses:u true
>>>
>>>     Performance counter stats for 'true':
>>>
>>>                    788  cache-misses
>>>                     95  LLC-load-misses
>>>
>>>            0.001025423  seconds time elapsed
>>>
>>> So it looks like you're right: in my case I have this cache
>>> configuration:
>>>
>> oops sorry, I replied too early...
>>
>> so my cache configuration is:
>>
>>     L1 ->  L2 ->  LLC
>>
>> where L2 misses is given by: 'cache-misses' - 'LLC-load-misses'
>>
>> Is that correct ?
>>
>> If so, I found 'cache-misses' term very not intuitive IMHO, probably
>> because I'm not an expert in cpu caches...
> Well thinking more about it, the above is wrong and I'm lost.
>
> If 'cache-misses' is the last level cache misses then how to interpret
> these results ?
>
>    $ perf stat -e llc-load-misses:u,llc-store-misses:u true
>
>     Performance counter stats for 'true':
>
>                     94  LLC-load-misses
>                      0  LLC-store-misses
>
>            0.000981840  seconds time elapsed
>
>
>    $ perf stat -e cache-misses:u true
>
>     Performance counter stats for 'true':
>
>                    796  cache-misses
>
>            0.001345136  seconds time elapsed
>
> Here 'cache-misses' value is much more than llc misses one...
>
If you are measuring last level cache misses, I would recommend you to 
use a memory intensive application/benchmark instead of /bin/true, as 
otherwise there can be a significant variation between two runs.

Victor

-- 
------------------------------------------------------------------------

      Victor Jimenez Perez
      Barcelona Supercomputing Center
      Centro Nacional de Supercomputacion
      WWW: http://www.bsc.es
      e-mail: victor.javier@....es

------------------------------------------------------------------------


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