Re: [RFC][PATCH 0/6] perf: x86 RDPMC and RDTSC support

[William Cohen <wcohen@redhat.com>]

On 11/21/2011 10:02 AM, Vince Weaver wrote:
> On Mon, 21 Nov 2011, Peter Zijlstra wrote:
> 
>> These few patches implement x86 RDPMC support and add an extention to the self
>> monitoring data to also allow additional time updates using userspace TSC reads.
>>
>> There's a few loose ends, but it mostly seems to work.
> 
> I'll have to test these out.  
> 
> I have some low-level benchmarks I've been working on, you can see some 
> preliminary results here:
>    http://web.eecs.utk.edu/~vweaver1/projects/perf-events/benchmarks/
> 
> perf_events was lagging behind perfmon2 and perfctr in self-monitoring 
> overhead, but maybe these changes will help that.
> 
> perf_event overhead got a lot worse between 2.6.32 and 3.0, I'm mid-bisect 
> on that trying to isolate the cause.
> 
> Vince
> 

Hi Vince,

Any improvements in performance of self-monitoring would be welcomed. International Symposium on computer Architecture (ISCA 2011) has paper about a low overhead mechanism to read out the performance counters, LiMiT (http://castl.cs.columbia.edu/limit/index.php/LiMiT) that avoids using system calls to read the performance counter data. That paper shows a fair amount of overhead for PAPI.

-Will