linux-kernel - Re: [PATCH 2/2] perf: Mmap 512 kiB by default

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Date:	Thu, 31 Mar 2011 14:33:36 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/2] perf: Mmap 512 kiB by default

On Thu, Mar 31, 2011 at 10:09:52AM +0200, Peter Zijlstra wrote:
> On Thu, 2011-03-31 at 03:35 +0200, Frederic Weisbecker wrote:
> > The default setting of perf record is to mmap 128 pages if the user
> > did not override with -m.
> > However the page size may vary accross different architecture
> > settings, giving different default size between each.
> > 
> > Moreover the kernel side still has a default max number of mlocked
> > pages of 512 kiB + 1 page for unprivileged users. 128 + 1 pages
> > with page size > 4096 overlaps this threshold.
> > 
> > Thus, better adapt to this limitation and set the default number of
> > pages to fit those 512 kiB + 1 page.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Cc: Stephane Eranian <eranian@...gle.com>
> > ---
> >  tools/perf/builtin-record.c |    6 +++++-
> >  1 files changed, 5 insertions(+), 1 deletions(-)
> > 
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index 6febcc1..a7e14bd 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -41,7 +41,7 @@ static u64			user_interval			= ULLONG_MAX;
> >  static u64			default_interval		=      0;
> >  
> >  static unsigned int		page_size;
> > -static unsigned int		mmap_pages			=    128;
> > +static unsigned int		mmap_pages			= UINT_MAX;
> >  static unsigned int		user_freq 			= UINT_MAX;
> >  static int			freq				=   1000;
> >  static int			output;
> > @@ -506,6 +506,10 @@ static int __cmd_record(int argc, const char **argv)
> >  	if (have_tracepoints(&evsel_list->entries))
> >  		perf_header__set_feat(&session->header, HEADER_TRACE_INFO);
> >  
> > +	/* 512 kiB: default amount of unprivileged mlocked memory */
> > +	if (mmap_pages == UINT_MAX)
> > +		mmap_pages = (512 * 1024) / page_size;
> > +
> >  	if (forks) {
> >  		child_pid = fork();
> >  		if (child_pid < 0) {
> 
> Ok, these two patches look good, I'll queue them. However a follow up
> might be to change the perf-record parameter from nr_pages to kb, which
> is a much more user friendly interface anyway ;-)

Agreed. I just wanted to keep that -m around on top of nr_pages beacuse
we have some tools/scripts relying on it, like perf lock, which need to
be changed as well.

We need a new option for this I think, and deprecate -m later perhaps, the
time for external users to migrate? Or just remove -m but it's certainly
going to be replaced soon.

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