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Date: Tue, 12 Nov 2013 12:07:51 -0300
From: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Ahern <dsahern@gmail.com>, linux-kernel@vger.kernel.org,
        mingo@kernel.org, jolsa@redhat.com,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Namhyung Kim <namhyung@kernel.org>, Mike Galbraith <efault@gmx.de>,
        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5
Message-ID: <20131112150751.GA19321@ghostprotocols.net>
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Em Tue, Nov 12, 2013 at 03:57:07PM +0100, Peter Zijlstra escreveu:
> On Tue, Nov 12, 2013 at 07:46:56AM -0700, David Ahern wrote:
> > When recording raw_syscalls for the entire system, e.g.,
> >     perf record -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
> > 
> > you end up with a negative feedback loop as perf itself calls write() fairly
> > often. This patch handles the problem by mmap'ing the file in chunks of 64M at
> > a time and copies events from the event buffers to the file avoiding write
> > system calls.
> 
> You know this completely fails the moment you trace faults, because
> every new access to one of those pages (to mark it dirty) will trigger a
> fault. And we'll take a bunch more faults -- one for each page -- than
> we ever did write() syscalls.

So we should provide a neon lettered warning when doing that, no? :-)
 
> Anyway the over all performance improvements still make it worth it. But
> the above seems like a false argument in favour of this.