Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2

[Ingo Molnar <mingo@elte.hu>]

* arun@sharma-home.net <arun@sharma-home.net> wrote:

> On Fri, Apr 22, 2011 at 12:52:11PM +0200, Ingo Molnar wrote:
> > 
> > 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!
> 
> One could argue that all you need is cycles and instructions. [...]

Yes, and note that with instructions events we even have skid-less PEBS 
profiling so seeing the precise .

> [...] If there is an expensive load, you'll see that the load instruction 
> takes many cycles and you can infer that it's a cache miss.
> 
> Questions app developers typically ask me:
> 
> * If I fix all my top 5 L3 misses how much faster will my app go?

This has come up: we could add a 'stalled/idle-cycles' generic event - i.e. 
cycles spent without performing useful work in the pipelines. (Resource-stall 
events on Intel CPUs.)

Then you would profile L3 misses (there's a generic event for that), plus 
stalls, and the answer to your question would be the percentage of hits you get 
in the stalled-cycles profile, multiplied by the stalled-cycles/cycles ratio.

> * Am I bottlenecked on memory bandwidth?

This would be a variant of the measurement above: say the top 90% of L3 misses 
profile-correlated with stalled-cycles, relative to total-cycles. If you get 
'90% of L3 misses cause a 1% wall-time slowdown' then you are not memory 
bottlenecked. If the answer is '35% slowdown' then you are memory bottlenecked.

> * I have 4 L3 misses every 1000 instructions and 15 branch mispredicts per
>   1000 instructions. Which one should I focus on?

AFAICS this would be another variant of stalled-cycles measurements: you create 
a stalled-cycles profile and check whether the top hits are branches or memory 
loads.

> It's hard to answer some of these without access to all events.

I'm curious, how would you measure these properties - do you have some 
different events in mind?

> While your approach of having generic events for commonly used counters might 
> be useful for some use cases, I don't see why exposing all vendor defined 
> events is harmful.
> 
> A clear statement on the last point would be helpful.

Well, the thing is, i think users are helped most if we add useful, highlevel 
PMU features added and not just an opaque raw event pass-through engine. The 
problem with lowlevel raw ABIs is that the tool space fragments into a zillion 
small hacks and there's no good concentration of know-how. I'd like the art of 
performance measurement to be generalized out, as well as it can be.

We had this discussion in the big perf-counters flamewars 2+ years ago, where 
one side wanted raw events, while we wanted intelligent kernel-side 
abstractions and generalizations. I think the abstraction and generalization 
angle worked out very well in practice - but we are having this discussion 
again and again :-)

As i stated it in my prior mails, i'm not against raw events as a rare 
exception channel - that increases utility. I'm against what was attempted 
here: an extension to raw events as the *primary* channel for DRAM measurement 
features. That is just sloppy and *reduces* utility.

I'm very simple-minded: when i see reduced utility i become sad :)

Thanks,

	Ingo