Re: [PATCH 4/5] perf record: mmap output file - v5

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>,
	linux-kernel@vger.kernel.org, jolsa@redhat.com,
	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
Date: Tue, 19 Nov 2013 12:48:10 +0100	[thread overview]
Message-ID: <20131119114810.GK3866@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20131119065835.GB32367@gmail.com>

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.