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

Elliott, Robert (Persistent Memory)
2018-11-27T00:12:28Z

> -----Original Message-----
> From: Daniel Jordan [mailto:daniel.m.jordan@oracle.com]
> Sent: Monday, November 19, 2018 10:02 AM
> 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
> > >
> > > 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?

I ran a short test with:
* HPE ProLiant DL360 Gen9 system
* Intel Xeon E5-2699 CPU with 18 physical cores (0-17) and
18 hyperthreaded cores (36-53)
* DDR4 NVDIMM-Ns (which run at regular DRAM DIMM speeds)
* fio workload generator
* cores on one CPU socket talking to a pmem device on the same CPU
* large (1 MiB) random writes (to minimize the threads getting CPU cache
hits from each other)

Results:
* 31.7 GB/s four threads, four physical cores (0,1,2,3)
* 22.2 GB/s four threads, two physical cores (0,1,36,37)
* 21.4 GB/s two threads, two physical cores (0,1)
* 12.1 GB/s two threads, one physical core (0,36)
* 11.2 GB/s one thread, one physical core (0)

So, I think it's important that the initialization threads run on
separate physical cores.

For the number of cores to use, one approach is:
memory bandwidth (number of interleaved channels * speed)
divided by
CPU core max sustained write bandwidth

For example, this 2133 MT/s system is roughly:
68 GB/s (4 * 17 GB/s nominal)
divided by
11.2 GB/s (one core's performance)
which is
6 cores

ACPI HMAT will report that 68 GB/s number. I'm not sure of
a good way to discover the 11.2 GB/s number.

fio job file:
[global]
direct=1
ioengine=sync
norandommap
randrepeat=0
bs=1M
runtime=20
time_based=1
group_reporting
thread
gtod_reduce=1
zero_buffers
cpus_allowed_policy=split
# pick the desired number of threads
numjobs=4
numjobs=2
numjobs=1

# CPU0: cores 0-17, hyperthreaded cores 36-53
[pmem0]
filename=/dev/pmem0
# pick the desired cpus_allowed list
cpus_allowed=0,1,2,3
cpus_allowed=0,1,36,37
cpus_allowed=0,36
cpus_allowed=0,1
cpus_allowed=0
rw=randwrite

Although most CPU time is in movnti instructions (non-temporal stores),
there is overhead in clearing the page cache and in the pmem block
driver; those won't be present in your initialization function.
perf top shows:
82.00% [kernel] [k] memcpy_flushcache
5.23% [kernel] [k] gup_pgd_range
3.41% [kernel] [k] __blkdev_direct_IO_simple
2.38% [kernel] [k] pmem_make_request
1.46% [kernel] [k] write_pmem
1.29% [kernel] [k] pmem_do_bvec

---
Robert Elliott, HPE Persistent Memory