Re: [PATCH 5/5] selftests/resctrl: Reduce failures due to outliers in MBA/MBM tests

[Ilpo Järvinen]

On Tue, 12 Sep 2023, Reinette Chatre wrote:
> On 9/11/2023 4:19 AM, Ilpo Järvinen wrote:
> > 5% difference upper bound for success is a bit on the low side for the
> 
> "a bit on the low side" is very vague.

The commit that introduced that 5% bound plainly admitted it's "randomly 
chosen value". At least that wasn't vague, I guess. :-)

So what I'm trying to do here is to have "randomly chosen value" replaced 
with a value that seems to work well enough based on measurements on 
a large set of platforms.

Personally, I don't care much about this, I can just ignore the failures 
due to outliers (and also reports about failing MBA/MBM test if somebody 
ever sends one to me), but if I'd be one running automated tests it would 
be annoying to have a problem like this unaddressed.

> > MBA and MBM tests. Some platforms produce outliers that are slightly
> > above that, typically 6-7%.
> > 
> > Relaxing the MBA/MBM success bound to 8% removes most of the failures
> > due those frequent outliers.
> 
> This description needs more context on what issue is being solved here.
> What does the % difference represent? How was new percentage determined?
> 
> Did you investigate why there are differences between platforms? From
> what I understand these tests measure memory bandwidth using perf and
> resctrl and then compare the difference. Are there interesting things 
> about the platforms on which the difference is higher than 5%?

Not really I think. The number just isn't that stable to always remain 
below 5% (even if it usually does).

Only systematic thing I've come across is that if I play with the read 
pattern for defeating the hw prefetcher (you've seen a patch earlier and 
it will be among the series I'll send after this one), it has an impact 
which looks more systematic across all MBM/MBA tests. But it's not what 
I'm trying now address with this patch.

> Could
> those be systems with multiple sockets (and thus multiple PMUs that need
> to be setup, reset, and read)? Can the reading of the counters be improved
> instead of relaxing the success criteria? A quick comparison between
> get_mem_bw_imc() and get_mem_bw_resctrl() makes me think that a difference
> is not surprising ... note how the PMU counters are started and reset
> (potentially on multiple sockets) at every iteration while the resctrl
> counters keep rolling and new values are just subtracted from previous.

Perhaps, I can try to look into it (add to my todo list so I won't 
forget). But in the meantime, this new value is picked using a criteria 
that looks better than "randomly chosen value". If I ever manage to 
address the outliers, the bound could be lowered again.

I'll update the changelog to explain things better.


-- 
 i.