linux-kernel - Re: [PATCH 4/5] perf record: mmap output file

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Message-ID: <20131119114810.GK3866@twins.programming.kicks-ass.net>
Date:	Tue, 19 Nov 2013 12:48:10 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	David Ahern <dsahern@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, jolsa@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5

On Tue, Nov 19, 2013 at 07:58:35AM +0100, Ingo Molnar wrote:
> 
> * David Ahern <dsahern@...il.com> wrote:
> 
> > This is mmap'ed output, not the ring buffers or its stack. As the 
> > output file grows, new pages are needed and those are allocated on 
> > access via page faults. The ftruncate only extends the file size, it 
> > does not allocate pages at that time.
> 
> Hm, doesn't MAP_POPULATE prefault pages in this case as well? 
> Prefaulting would avoid the most obvious page fault driven feedback 
> loops and it would probably be faster as well, because it avoids all 
> the pagefaults ...

MAP_POPULATE does indeed seem to write pre-fault for writable maps
(mm/mlock.c:__mlock_vma_pages_range).

And that does indeed seem to side-step the perf sw pagefault event, but
that is arguably a perf bug.

If we were to fix this, MAP_POPULATE would simply generate a metric ton
of events, we'd have to write out with yet more MAP_POPULATE.
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