linux-kernel - Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units

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Date:	Wed, 18 Dec 2013 15:11:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow
 tracing units

On Wed, Dec 18, 2013 at 04:01:04PM +0200, Alexander Shishkin wrote:
> > Why don't you start by explaining _why_ you need a second stream to
> > begin with?
> 
> Oh, I'm sure I've explained it earlier ([1], [2])

See, I didn't read 0 because that information gets lost and patches
should be self explanatory, and i didn't get to the Intel driver yet
because well, I got stuck in the generic code.

> but why not. The data
> in the second stream is generated at a rate which is hundreds of
> megabytes per second per core. Decoding this data is ~1000 times slower
> than generating it. Ergo, can't be done in kernel, needs to be exported
> as-is to userspace for later retreival and decoding. Doing it via perf
> stream means an extra copy, which at these rates is a waste. Ergo, a
> second buffer.

Still confused, if you cannot copy it into one buffer, then why can you
copy it into a second buffer?

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