Re: [perf] howto switch from pfmon

From: Ingo Molnar
Date: Tue Jun 23 2009 - 09:25:28 EST

* Ingo Molnar <mingo@xxxxxxx> wrote:

> > I guess there are still a lot of things on the TODOlist but I'd
> > like to understand a bit more where things are going. Sorry I
> > didn't read all the archives about this, there are way too many
> > of them recently :)
> Yeah, there's indeed still a lot on the TODO list :-)
> CPU_TO_DRAM_REQUESTS_TO_TARGET_NODE is a Barcelona hardware event,
> so if you know that it maps to raw ID 0x100000e0 then you can
> always extend the events that 'perf' knows about via raw events:
> $ perf stat -e cycles -e instructions -e r1000ffe0 ./hackbench 10

Note, beyond using raw events, if you are interested in profiling
out 'locality badness' of your app, you are probably quite well
served with the default metrics on Barcelona as well:

$ perf stat ~/hackbench 10
Time: 0.205

Performance counter stats for '/home/mingo/hackbench 10':

2187.328436 task-clock-msecs # 3.315 CPUs
54554 context-switches # 0.025 M/sec
1160 CPU-migrations # 0.001 M/sec
17755 page-faults # 0.008 M/sec
4995437535 cycles # 2283.808 M/sec
2150881875 instructions # 0.431 IPC
644099534 cache-references # 294.469 M/sec
8516562 cache-misses # 3.894 M/sec

0.659895237 seconds time elapsed.

The cache-misses event is sufficiently well-represented to be
meaningful to profile based on it. Raw DRAM access stats can be
useful too - but they are generally layered much later and your app
can hurt already flip-flopping its working set, without hitting too
hard on the DRAM channels.

So perhaps 'cache-misses' is a good first-level approximation metric
to measure and profile along. You can get a good
(last-level-)cache-misses profile using the auto-freq counters:

perf record -e cache-misses -F 10000 ./your-app

The '-F 10000' tells the kernel to do 10 KHz sampling of your-app,
regardless of how frequent cache-misses are. The tools (perf report)
will take the weight of events into account, so it's all
well-normalized between the functions.

So you dont need to specify the 'sampling interval' by hand to get a
sufficient number of samples, you just specify a sampling frequency
- and the perfcounters subsystem takes care of the rest.

Also, your system wont over-sample nor under-sample if your workload
idles around occasionally.

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