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

[David Ahern]

On 11/12/13, 8:19 AM, Peter Zijlstra wrote:
> On Tue, Nov 12, 2013 at 12:07:51PM -0300, Arnaldo Carvalho de Melo wrote:
> > 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? :-)
>
> Dunno.. it _should_ all work. Try it and see what it does.. Once the
> events are bigger than a page things might get 'interesting' though.

one option here is not allow page faults and system wide system calls. 
system wide tracing needs mmap; page faults for a task can use write(). 
I left that option in case something like this came up.

David
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