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Date: Tue, 19 Nov 2013 12:48:10 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>, Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
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        Frederic Weisbecker <fweisbec@gmail.com>,
        Mike Galbraith <efault@gmx.de>, Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5
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On Tue, Nov 19, 2013 at 07:58:35AM +0100, Ingo Molnar wrote:
> 
> * David Ahern <dsahern@gmail.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.