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Date: Fri, 22 Apr 2011 14:04:53 +0200
From: Stephane Eranian <eranian@...gle.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: Arnaldo Carvalho de Melo <acme@...radead.org>,
linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>,
Lin Ming <ming.m.lin@...el.com>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>, eranian@...il.com,
Arun Sharma <asharma@...com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add
missing user space support for config1/config2
On Fri, Apr 22, 2011 at 12:52 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Stephane Eranian <eranian@...gle.com> wrote:
>
>> >> Generic cache events are a myth. They are not usable. [...]
>> >
>> > Well:
>> >
>> > aldebaran:~> perf stat --repeat 10 -e instructions -e L1-dcache-loads -e L1-dcache-load-misses -e LLC-misses ./hackbench 10
>> > Time: 0.125
>> > Time: 0.136
>> > Time: 0.180
>> > Time: 0.103
>> > Time: 0.097
>> > Time: 0.125
>> > Time: 0.104
>> > Time: 0.125
>> > Time: 0.114
>> > Time: 0.158
>> >
>> > Performance counter stats for './hackbench 10' (10 runs):
>> >
>> > 2,102,556,398 instructions # 0.000 IPC ( +- 1.179% )
>> > 843,957,634 L1-dcache-loads ( +- 1.295% )
>> > 130,007,361 L1-dcache-load-misses ( +- 3.281% )
>> > 6,328,938 LLC-misses ( +- 3.969% )
>> >
>> > 0.146160287 seconds time elapsed ( +- 5.851% )
>> >
>> > It's certainly useful if you want to get ballpark figures about cache behavior
>> > of an app and want to do comparisons.
>> >
>> What can you conclude from the above counts?
>> Are they good or bad? If they are bad, how do you go about fixing the app?
>
> So let me give you a simplified example.
>
> Say i'm a developer and i have an app with such code:
>
> #define THOUSAND 1000
>
> static char array[THOUSAND][THOUSAND];
>
> int init_array(void)
> {
> int i, j;
>
> for (i = 0; i < THOUSAND; i++) {
> for (j = 0; j < THOUSAND; j++) {
> array[j][i]++;
> }
> }
>
> return 0;
> }
>
> Pretty common stuff, right?
>
> Using the generalized cache events i can run:
>
> $ perf stat --repeat 10 -e cycles:u -e instructions:u -e l1-dcache-loads:u -e l1-dcache-load-misses:u ./array
>
> Performance counter stats for './array' (10 runs):
>
> 6,719,130 cycles:u ( +- 0.662% )
> 5,084,792 instructions:u # 0.757 IPC ( +- 0.000% )
> 1,037,032 l1-dcache-loads:u ( +- 0.009% )
> 1,003,604 l1-dcache-load-misses:u ( +- 0.003% )
>
> 0.003802098 seconds time elapsed ( +- 13.395% )
>
> I consider that this is 'bad', because for almost every dcache-load there's a
> dcache-miss - a 99% L1 cache miss rate!
>
> Then i think a bit, notice something, apply this performance optimization:
>
I don't think this example is really representative of the kind of problems
people face, it is just too small and obvious. So I would not generalize on it.
If you are happy with generalized cache events then, as I said, I am fine with
it. But the API should ALWAYS allow users access to raw events when they
need finer grain analysis.
> diff --git a/array.c b/array.c
> index 4758d9a..d3f7037 100644
> --- a/array.c
> +++ b/array.c
> @@ -9,7 +9,7 @@ int init_array(void)
>
> for (i = 0; i < THOUSAND; i++) {
> for (j = 0; j < THOUSAND; j++) {
> - array[j][i]++;
> + array[i][j]++;
> }
> }
>
> I re-run perf-stat:
>
> $ perf stat --repeat 10 -e cycles:u -e instructions:u -e l1-dcache-loads:u -e l1-dcache-load-misses:u ./array
>
> Performance counter stats for './array' (10 runs):
>
> 2,395,407 cycles:u ( +- 0.365% )
> 5,084,788 instructions:u # 2.123 IPC ( +- 0.000% )
> 1,035,731 l1-dcache-loads:u ( +- 0.006% )
> 3,955 l1-dcache-load-misses:u ( +- 4.872% )
>
> - I got absolute numbers in the right ballpark figure: i got a million loads as
> expected (the array has 1 million elements), and 1 million cache-misses in
> the 'bad' case.
>
> - I did not care which specific Intel CPU model this was running on
>
> - I did not care about *any* microarchitectural details - i only knew it's a
> reasonably modern CPU with caching
>
> - I did not care how i could get access to L1 load and miss events. The events
> were named obviously and it just worked.
>
> So no, kernel driven generalization and sane tooling is not at all a 'myth'
> today, really.
>
> So this is the general direction in which we want to move on. If you know about
> problems with existing generalization definitions then lets *fix* them, not
> pretend that generalizations and sane workflows are impossible ...
>
Again, to fix them, you need to give us definitions for what you expect those
events to count. Otherwise we cannot make forward progress.
Let me give just one simple example: cycles
What your definition for the generic cycle event?
There are various flavors:
- count halted, unhalted cycles?
- impacted by frequency scaling?
LLC-misses:
- what considered the LLC?
- does it include code, data or both?
- does it include demand, hw prefetch?
- it is to local or remote dram?
Once you have clear and precise definition, then we can look at the actual
events and figure out a mapping.
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