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

[Daniel Jordan <daniel.m.jordan@oracle.com>]

On Mon, Nov 12, 2018 at 10:15:46PM +0000, Elliott, Robert (Persistent Memory) wrote:
> 
> 
> > -----Original Message-----
> > From: Daniel Jordan <daniel.m.jordan@oracle.com>
> > Sent: Monday, November 12, 2018 11:54 AM
> > To: Elliott, Robert (Persistent Memory) <elliott@hpe.com>
> > Cc: Daniel Jordan <daniel.m.jordan@oracle.com>; linux-mm@kvack.org;
> > kvm@vger.kernel.org; linux-kernel@vger.kernel.org; aarcange@redhat.com;
> > aaron.lu@intel.com; akpm@linux-foundation.org; alex.williamson@redhat.com;
> > bsd@redhat.com; darrick.wong@oracle.com; dave.hansen@linux.intel.com;
> > jgg@mellanox.com; jwadams@google.com; jiangshanlai@gmail.com;
> > mhocko@kernel.org; mike.kravetz@oracle.com; Pavel.Tatashin@microsoft.com;
> > prasad.singamsetty@oracle.com; rdunlap@infradead.org;
> > steven.sistare@oracle.com; tim.c.chen@intel.com; tj@kernel.org;
> > vbabka@suse.cz
> > 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@vger.kernel.org <linux-kernel-
> > > > owner@vger.kernel.org> 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