> -----Original Message----- > From: Daniel Jordan [mailto:daniel.m.jordan@xxxxxxxxxx] > 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@xxxxxxxxxx> > > > 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@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? 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
