Re: [PerfSuite-users] bad statistics from psprocess on Altix 3700

[Rick K. <rk...@nc...>]

Phuong,

> I was trying to use psrun / psprocess to look at a code segment and got
> the following bad statistics from psprocess
> 
> Level 2 cache line reuse (data)........................................               -0.999
> 
> Level 2 cache hit rate (data)..........................................            -1494.576
> 
> All the other stat seem reasonable.  Any idea why ?

This is a pretty common thing to see in psprocess output when using an 
IA-64 platform like the Altix.  The statistics that are calculated by 
psprocess are pretty "generic", to be used with PAPI events.  They are 
taken from an XML database that should be installed in:

PREFIX/share/perfsuite/xml/pshwpc/PAPI_metrics.xml

The definition of the hit rate above uses PAPI_L1_DCM and PAPI_L2_DCM.  On 
the Itanium 2, floating point loads bypass the L1 cache with the result 
that  you can have more L2 cache misses than L1 cache misses (you can see 
if this is the case by looking either at the raw XML output or the display 
earlier on in the psprocess summary).  This is a bit counter-intuitive and 
is what I think you may be seeing here (if not, please forward along one 
of the .xml files that were generated).

I think there are ways to qualify things further with the Itanium 2 PMU to 
correct for this, but this is not done in PerfSuite, so one has to look at 
some of these statistics in the context of what's going on underneath.

Rick