Re: [mm] 2d146aa3aa: vm-scalability.throughput -36.4% regression

[Andi Kleen]

On 9/1/2021 6:35 PM, Feng Tang wrote:
> On Wed, Sep 01, 2021 at 08:12:24AM -0700, Andi Kleen wrote:
> > Feng Tang <feng.tang@xxxxxxxxx> writes:
> > > Yes, the tests I did is no matter where the 128B padding is added, the
> > > performance can be restored and even improved.
> > I wonder if we can find some cold, rarely accessed, data to put into the
> > padding to not waste it. Perhaps some name strings? Or the destroy
> > support, which doesn't sound like its commonly used.
> Yes, I tried to move 'destroy_work', 'destroy_rwork' and 'parent' over
> before the 'refcnt' together with some padding, it restored the performance
> to about 10~15% regression. (debug patch pasted below)
>
> But I'm not sure if we should use it, before we can fully explain the
> regression.

Narrowing it down to a single prefetcher seems good enough to me. The 
behavior of the prefetchers is fairly complicated and hard to predict, 
so I doubt you'll ever get a 100% step by step explanation.


-Andi