Re: [PATCH 10/10] perf record: Add an option to force per-cpu mmaps

[Adrian Hunter]

On 15/11/13 14:05, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
>> On Fri, Nov 15, 2013 at 12:56:29PM +0100, Ingo Molnar wrote:
>>> So, here's the current status quo, there's 4 basic types of profiling 
>>> that 99% of the people are using, in order of popularity:
>>>
>>> 	perf record <cmd>
>>> 	perf record -a sleep N
>>> 	perf record -p <PID>
>>> 	perf record -t <TID>
>>>
>>> The first two (which I'd guess comprise about 95% of real-world usage) 
>>> have inheritance enabled.
>>>
>>> The last two (-p/-t) have inheritance disabled by default.
>>
>> Yes, and I would expect it to be disabled for the TID option as you 
>> explicitly select a single threads.
> 
> Correct.
> 
>> For the process wide thing it would make sense to enable inheritance 
>> by default though.
>>
>> So the big trade-off is that for single threaded processes which do 
>> not fork you now have a single buffer, whereas with the inheritance 
>> option you'll end up with nr_cpus buffers by default.
>>
>> I suppose for most normal people that's not really an issue; and I 
>> suppose all people with silly large machines already pay extra 
>> attention -- but at least make it explicit and very clear that this 
>> is so.
> 
> Do the first variant, 'perf record <cmd>', already use per CPU 
> buffers?

Yes.

Another difference (and I need to fix the patch) is that per-cpu mmaps
require PERF_SAMPLE_TIME so that the events do not appear out of order.

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