linux-kernel - Re: perf tools miscellaneous questions

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Date:	Mon, 08 Nov 2010 20:43:48 +0100
From:	Francis Moreau <francis.moro@...il.com>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	Victor Jimenez <victor.javier@....es>,
	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

Vince Weaver <vweaver1@...s.utk.edu> writes:

> This is rapidly getting of topic, especially for linux-kernel

Don't think so but feel free to remove LKML from Cc.

[...]

> Most events are poorly documented, if at all.  And the Linux kernel 
> predefined event list is loosely based upon the intel architectural
> events, which not every processor has and I've heard from insiders saying 
> that you should be very careful for the results from those events.

I agree, that's why I try to clarify some events.

Perf tools are cool stuffs, IMHO, but it's pretty hard for me to
interpret results. I tried to compare some numbers in my previous posts
but I got some 'random' figures for now.

Another example is given below where I'm trying to bench a 2 functions
which do the same thing but differently.

   $ perf stat -e cache-misses:u,l1d-loads-misses:u,cycles:u -p $(pgrep test)
     C-c C-c
    Performance counter stats for process id '30263':
   
                406532  cache-misses            
               4986030  L1-dcache-load-misses   
             120247366  cycles                  
   
           2.482196928  seconds time elapsed
   
   
   $ perf stat -e cache-misses:u,l1d-loads-misses:u,cycles:u -p $(pgrep test)
     C-c C-c
    Performance counter stats for process id '30271':
   
                459683  cache-misses            
               2513338  L1-dcache-load-misses   
             159968076  cycles                  
   
           2.129021265  seconds time elapsed

Which numbers are important here ? cache-misses ? L1-dcache-load-misses
?

I just can say that the first run looks faster.

-- 
Francis
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