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SubjectRe: [patch] Performance Counters for Linux, v4
Vince Weaver wrote:
> I'm trying to evaluate this new proposal for the kind of workloads I use
> performance counters for, and even the simplest tests don't work.
> I'm trying to do a simple aggragate count for some benchmarks here using
> timec and I'm getting poor results.
> Are any of the problems I'm reporting going to be fixed?
> In any case, I was testing aggregate counts on a longer running
> benchmark, this time equake from the spec2k benchmark suite, still on
> the q6600.
> If I only count retired instructions, I get consistent results:
> timec -e 1
> 119175255369 instructions (events)
> 119175255561 instructions (events)
> 119175255383 instructions (events)
> however the minute I add another count, say cycles so I can calculate
> CPI/IPC the results for instructions are suddenly off by 33%.
> Needless to say, perfmon can handle reading both cycles and instructions
> at the same time.
> timec -e 0, -e 1
> 91758816320 cycles (events)
> 79428247907 instructions (events)
> 91849140396 cycles (events)
> 79449560742 instructions (events)
> It gets worse when trying to look at cache statistics:
> timec -e 1 -e 2 -e 3
> 59611457943 instructions (events)
> 1872499771 cache references (events)
> 97471971 cache misses (events)
> 59601907232 instructions (events)
> 1871766376 cache references (events)
> 97435199 cache misses (events)
> and so on
> timec -e1 -e2 -e3 -e4
> 47671703285 instructions (events)
> 1498246999 cache references (events)
> 77838085 cache misses (events)
> 3394839360 branches (events)
> 47666131604 instructions (events)
> 1497069685 cache references (events)
> 78065325 cache misses (events)
> 3393244879 branches (events)
> So apparently this performance counter infrastructure will always be
> useless for trying to get plain aggregate counts? It's the simplest
> case to get right, so it makes me wonder about the design of the rest of
> the infrastructure.
> Vince

Your test case demonstrates that scaling is missing from the current
version of Performance Counters for Linux.

When each set of events is scheduled onto a set of hardware event
counters, in order to scale the results properly, a cycles counter needs
to be included in each set as well.

When the counts are read up, the counts from each set need to be scaled
by a factor of
(total cycles)/(cycles in that set)

This is something that can be handled by perfmon3 (full) because set
multiplexing is explicitly programmed, not transparent as it is in
Ingo's current code. In perfmon3, the set switching can be determined
by events counter overflow, as well as time.

In common with both perfmon3 and Ingo's solution is that as more and
more events are scheduled onto the same set of hardware registers, the
accuracy drops and has to be compensated with longer run times.

Another source of error is that if the sets are rotated across the
hardware at a fixed periodic rate, if there's any correlation between
that rate and what's going on in the program being analyzed, the results
will be dubious. Ideally, you'd want to have some sort of pseudo-random
set switching rate to mitigate this sort of problem.

If Ingo could make some sort of provision for including a cycles count
in every set, and then transparently performing the scaling, that would
make it easier to use. As it stands now, I don't think there's any way
to recover the needed scaling information, because you cannot tell what
events are in what sets and how many cycles are associated with each set.

- Corey

Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR

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