Re: [PATCH v4 10/11] perf/x86/intel: Perform rotation on Intel CQM RMIDs

[Peter Zijlstra]

On Fri, Jan 09, 2015 at 12:55:07PM +0000, Matt Fleming wrote:
> On Wed, 07 Jan, at 01:16:17PM, Peter Zijlstra wrote:
> > On Fri, Nov 14, 2014 at 09:15:11PM +0000, Matt Fleming wrote:
> >  +	/*
> > > +	 * A reasonable upper limit on the max threshold is the number
> > > +	 * of lines tagged per RMID if all RMIDs have the same number of
> > > +	 * lines tagged in the LLC.
> > > +	 *
> > > +	 * For a 35MB LLC and 56 RMIDs, this is ~1.8% of the LLC.
> > > +	 */
> > > +	__intel_cqm_max_threshold =
> > > +		boot_cpu_data.x86_cache_size * 1024 / (cqm_max_rmid + 1);
> > 
> > Seeing how a percentage is without unit, the 35MB figure seems
> > pointless.
>  
> It's only an example to demonstrate that this fudge calculation makes
> sense on the current class of CQM-enabled hardware.
> 
> > Also, why would a flat distribution be a good measure for 'empty'? I
> > would think that would in fact constitute in use.
>  
> It's not, it's a good measure for 'full'. This is the *max* threshold.
> When searching for RMIDs to stabilize we'll stop searching if
> __intel_cqm_threshold == __intel_cqm_max_threshold, since that indicates
> all our RMIDs have *so* many lines tagged that it's unlikely increasing
> __intel_cqm_threshold any further would be a win.

Right, but we'll also consider RMIDs with less than this as fit for
reuse. So we'll re-use RMIDs that are effectively full.

Our aim is to acquire an 'empty' RMID, not give up and start reusing
full ones just because, right?
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