Re: Skylake minimal counts

On Mon, 4 Dec 2017, William Cohen wrote:
> On 12/04/2017 11:19 AM, Michael Petlan wrote:
> > Hi all!
> > 
> > During testing on a Skylake-SP, I have noticed that most of the Skylake
> > events have "minimum:2000003" set, even for events that used to have much
> > lower minimum counts on various older micro-architectures, such as Haswell,
> > Ivy Bridge, etc.
> > 
> > An example might be cpu_clk_unhalted which has usually 6000, except of the
> > following models:
> > 
> > Goldmont:     minimum:2000003
> > Skylake:      minimum:2000003
> > Westmere:     minimum:100000
> > 
> > Similarly, for inst_retired, for previous CPUs, it was usually 6000, except of:
> > 
> > Broadwell:    minimum:2000003
> > Goldmont:     minimum:2000003
> > Haswell:      minimum:2000003
> > Sandy Bridge: minimum:2000000
> > Skylake:      minimum:2000003
> > Westmere:     minimum:2000003
> > 
> > (e.g. Atom, Core2, Nehalem are 6000)
> 
> 
> Hi Michael,
> 
> Setting the clk_unhalted minimum to 6000 would be unwise.  The interrupt handler to collect a sample is on the order of a few thousand cycles.  The clk_unhalted and inst_retired should probably stay in the range they are in, maybe be a bit lower.  For infrequent events such as baclear limits in the millions would too high and require someone to wait a long time to get enough samples.  The general idea is want lower limit to limit the number of samples to around a thousand samples a second on a processor.
> 
I don't insist on 6000 for cpu_clk_unhalted. It's just what OProfile has for most
of the Intels... 

> -Will
> 
> 
> > 
> > I know that the minimal counts are nothing precise and were determined rather
> > "randomly", but for Skylakes, it seems that 2000003 is just the "default for
> > everything" except of some exceptions...
> > 
> > Since OProfile does not allow users to enforce a lower value there, the 2000003
> > values seem to be pretty limiting, don't they?
> > 
> > Shouldn't they be re-configured? If you think so, I might do that. However,
> > in such case, I am curious how would you do it (which load/criteria would you
> > use)...
> > 
> > Cheers,
> > Michael
> > 
> 
> 

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