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

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Nov 15, 2013 at 09:41:53AM -0700, David Ahern wrote:
> > On 11/13/13, 4:34 AM, Peter Zijlstra wrote:
> > >>>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.
> > >>
> > >>So maybe splice() sounds like the right long term solution after all? :-/
> > >
> > >Right until you put a tracepoint (kprobe) somewhere in whatever function
> > >is used to transfer a single page into/from a splice pipe.
> > >
> > >You can always screw yourself over using this stuff, no exceptions.
> > >
> > 
> > What now? Can we add the mmap path as an option?
> 
> I'd say an option is always a possibility, but someone please try 
> what happens if you use stupid large events (dwarf stack copies) on 
> PERF_COUNT_SW_PAGE_FAULTS (.period=1) while recording with mmap().
> 
> The other option is to simply disallow PERF_SAMPLE_STACK_USER for 
> that event.
> 
> Personally I think 8k copies for every event are way stupid anyway, 
> that's a metric ton of data at a huge cost.

Well, with 1 khz sampling of a single threaded workload it's 8MB per 
second - that's 80 MB for 10 seconds profiling - not the end of the 
world.

Thanks,

	Ingo