Re: [RFC PATCH v4 11/13] mm: parallelize deferred struct page initialization within each node

[Daniel Jordan]

On Mon, Nov 12, 2018 at 10:15:46PM +0000, Elliott, Robert (Persistent Memory) wrote:
> 
> 
> > -----Original Message-----
> > From: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > Sent: Monday, November 12, 2018 11:54 AM
> > To: Elliott, Robert (Persistent Memory) <elliott@xxxxxxx>
> > Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>; linux-mm@xxxxxxxxx;
> > kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; aarcange@xxxxxxxxxx;
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> > Subject: Re: [RFC PATCH v4 11/13] mm: parallelize deferred struct page
> > initialization within each node
> > 
> > On Sat, Nov 10, 2018 at 03:48:14AM +0000, Elliott, Robert (Persistent
> > Memory) wrote:
> > > > -----Original Message-----
> > > > From: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-
> > > > owner@xxxxxxxxxxxxxxx> On Behalf Of Daniel Jordan
> > > > Sent: Monday, November 05, 2018 10:56 AM
> > > > Subject: [RFC PATCH v4 11/13] mm: parallelize deferred struct page
> > > > initialization within each node
> > > >
> ...
> > > > In testing, a reasonable value turned out to be about a quarter of the
> > > > CPUs on the node.
> > > ...
> > > > +	/*
> > > > +	 * We'd like to know the memory bandwidth of the chip to
> > > >         calculate the
> > > > +	 * most efficient number of threads to start, but we can't.
> > > > +	 * In testing, a good value for a variety of systems was a
> > > >         quarter of the CPUs on the node.
> > > > +	 */
> > > > +	nr_node_cpus = DIV_ROUND_UP(cpumask_weight(cpumask), 4);
> > >
> > >
> > > You might want to base that calculation on and limit the threads to
> > > physical cores, not hyperthreaded cores.
> > 
> > Why?  Hyperthreads can be beneficial when waiting on memory.  That said, I
> > don't have data that shows that in this case.
> 
> I think that's only if there are some register-based calculations to do while
> waiting. If both threads are just doing memory accesses, they'll both stall, and
> there doesn't seem to be any benefit in having two contexts generate the IOs
> rather than one (at least on the systems I've used). I think it takes longer
> to switch contexts than to just turnaround the next IO.

(Sorry for the delay, Plumbers is over now...)

I guess we're both just waving our hands without data.  I've only got x86, so
using a quarter of the CPUs rules out HT on my end.  Do you have a system that
you can test this on, where using a quarter of the CPUs will involve HT?

Thanks,
Daniel