Re: [perfmon2] PEBS support update

[Carole Wu <cw...@gm...>]

Got it. Thanks very much for the explanation.

--Carole

On Sun, Nov 1, 2009 at 7:06 AM, stephane eranian <er...@go...>wrote:

> On Sun, Nov 1, 2009 at 4:37 AM, Carole Wu <cw...@gm...> wrote:
> > Hello,
> >
> > pfmon --smpl-module=pebs-ll -e MEM_INST_RETIRED:LATENCY_ABOVE_THRESHOLD \
> >          --ld-lat-threshold=4 --long-smpl-periods=2000 --smpl-compact
> > --with-header ...
> >
> > This should generate a trace of sample, where each sample represents the
> > 2000th long latency (>4 cycles) memory operations. 2000 x (the number of
> > samples in the trace) should equal to the number of
> LAST_LEVEL_CACHE_MISSES
> > collected with perfmon2's "counting" capability, e.g. pfmon -e
> > LAST_LEVEL_CACHE_MISSES. Is this right?
>
> No.
>
> First of all this is not tracing but statistical sampling. You are
> going to loose some misses.
> There are shadowing effects. In order to complete a sample, you need
> the latency. Until
> it is collected, no other loads can be sampled even if they exceed the
> threshold, i.e., PEBS
> can only track one load at a time. When the PEBS buffer fills up,
> monitoring stop, but execution
> continues, you are missing some more loads there.
>
> Note that this discrepancy is not specific to PEBS, you get the same
> behavior with AMD IBS or
> Itanium D-EAR. But the idea is that if you run for long enough, you
> will eventually get enough
> representative samples to approximate a trace.
>
>
> > I am seeing mismatching numbers for counts collected with PEBS and the
> > counting feature in perfmon2 for my Nehalem machine.
> > Your help is appreciated. Thanks,
> > Carole
> > On Wed, Oct 28, 2009 at 4:36 AM, stephane eranian <
> er...@go...>
> > wrote:
> >>
> >> Hi,
> >>
> >> I am happy to report that I have now uploaded all the code necessary to
> >> use
> >> PEBS on Intel Core, Atom, and Nehalem. That includes PEBS-LL on Nehalem
> >> which is used to sample where cache misses occur.
> >>
> >> What you need:
> >>    - latest libpfm sources from CVS
> >>
> >>    - latest pfmon sources from CVS
> >>
> >>    - perfmon2 2.6.30 from GIT
> >>
> >>      git clone
> >> git://git.kernel.org/pub/scm/linux/kernel/git/eranian/linux-2.6.git
> >>      Make sure you enabled 'Unified PEBS'
> >>
> >>
> >> This kernel includes a unified PEBS sampling format which supports
> >> Netburst,
> >> Core, Atom, and Nehalem. You must insert the module perfmon_pebs_smpl
> >> (or compile in the code).
> >>
> >> Next, to use PEBS, you can simply do:
> >>
> >>   pfmon --smpl-module=pebs --smpl-compact --with-header
> >> -einst_retired:any_p \
> >>             --long-smpl-period=2400000 ...
> >>
> >>   Not all events support PEBS. In --smpl-compact mode, each line
> >> contains a PEBS
> >>   sample.
> >>
> >> To collect cache misses on Nehalem, you can do:
> >>
> >> pfmon --smpl-module=pebs-ll -e MEM_INST_RETIRED:LATENCY_ABOVE_THRESHOLD
> \
> >>          --ld-lat-threshold=4 --long-smpl-periods=2000 --smpl-compact
> >> --with-header ...
> >>
> >>  You must use the MEM_INST_RETIRED:LATENCY_ABOVE_THRESHOLD to activate
> >> this
> >>  HW feature.
> >>
> >> Each line contains a PEBS record, including the cache miss
> >> information. The ld-lat parameter
> >> is the minimal threshold for the miss latency. Only misses >=
> >> threshold are captured. It must
> >> be at least 4. 4 cycles is the L1D hit latency. For each captured
> >> miss, you get an instruction addr,
> >> data addr, miss latency, source of the data (where did it come from,
> >> refer to Intel documentation).
> >> It is important to understand that the instruction addr does NOT point
> >> to the load instruction but
> >> ALWAYS to the next dynamic instruction, i.e., the whole state is
> >> recorded at retirement of the load.
> >>
> >>
> >> On Mon, Oct 5, 2009 at 1:55 AM, Carole Wu <cw...@gm...> wrote:
> >> > Hello,
> >> >
> >> > I'd like to collect information about my workload, running on Nehalem,
> >> > using
> >> > PEBS, so I use the following command.
> >> >
> >> >>> pfmon -e MEM_INST_RETIRED:LATENCY_ABOVE_THRESHOLD
> --ld-lat-threshold=1
> >> >>> --long-smpl-periods=2000 --short-smpl-periods=200 ./mcf_base inp.in
> >> >>> load latency threshold not yet supported
> >> > However, the response seems to suggest that my machine does not
> >> > currently
> >> > support PEBS? Is it true, or am I not setting parameters correctly?
> >> >
> >> > Any help is greatly appreciated.
> >> >
> >> > Carole
> >> >
> >> >
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