linux-kernel - Re: [PATCH 1/3] perf tools: Cache register accesses for unwind processing

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Date:	Tue, 29 Apr 2014 09:36:19 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Jean Pihet <jean.pihet@...aro.org>
Subject: Re: [PATCH 1/3] perf tools: Cache register accesses for unwind processing

On Mon, 28 Apr 2014 15:24:20 +0200, Jiri Olsa wrote:
> On Mon, Apr 28, 2014 at 10:02:55PM +0900, Namhyung Kim wrote:
>> Hi Jiri,
>> 
>> 2014-04-28 (월), 11:48 +0200, Jiri Olsa:
>> > On Sun, Apr 27, 2014 at 11:29:21PM +0900, Namhyung Kim wrote:
>> > > Hi Jiri,
>> > > 
>> > > 2014-04-17 (목), 19:39 +0200, Jiri Olsa:
>> > > > Caching registers value into an array. Got about 4% speed up
>> > > > of perf_reg_value function for report command processing
>> > > > dwarf unwind stacks.
>> > > 
>> > > I'm not familiar with the code base, so probably silly questions:  Where
>> > > does the speed up come from?  IOW I don't know what's the difference
>> > > between the regs->regs and regs->cached_regs.  And does the cached_regs
>> > > contain correct values of registers for each frame?
>> > 
>> > the current way register's value is accessed is to get its
>> > index in the sample's regs array.. based on register's id
>> > and the registers mask
>> > 
>> > so each time you want register value you traverse the registers
>> > mask and count reg's index for the sample regs array
>> > 
>> > this patch does this only once for each register (at the time it's
>> > first accessed) and cache its value in the array (cache_regs). The
>> > cache_mask is used to identify which regs are already cached.
>> 
>> That means it'll get the same value everytime it accesses a register in
>> frames in a sample?
>
> right..

Hmm.. I thought it'd be changed somehow as it unwinds frames.

Thanks,
Namhyung
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