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

[Arnaldo Carvalho de Melo <acme@ghostprotocols.net>]

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? :-)
 
> Anyway the over all performance improvements still make it worth it. But
> the above seems like a false argument in favour of this.