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

[David Ahern <dsahern@gmail.com>]

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