Messages in this thread | | | Date | Thu, 15 Oct 2020 11:08:43 +0200 | From | Jan Kara <> | Subject | Re: [mm/writeback] 8d92890bd6: will-it-scale.per_process_ops -15.3% regression |
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On Thu 15-10-20 08:46:01, NeilBrown wrote: > On Wed, Oct 14 2020, Jan Kara wrote: > > > On Wed 14-10-20 16:47:06, kernel test robot wrote: > >> Greeting, > >> > >> FYI, we noticed a -15.3% regression of will-it-scale.per_process_ops due > >> to commit: > >> > >> commit: 8d92890bd6b8502d6aee4b37430ae6444ade7a8c ("mm/writeback: discard > >> NR_UNSTABLE_NFS, use NR_WRITEBACK instead") > >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master > > > > Thanks for report but it doesn't quite make sense to me. If we omit > > reporting & NFS changes in that commit (which is code not excercised by > > this benchmark), what remains are changes like: > > > > nr_pages += node_page_state(pgdat, NR_FILE_DIRTY); > > - nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS); > > nr_pages += node_page_state(pgdat, NR_WRITEBACK); > > ... > > - nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) + > > - global_node_page_state(NR_UNSTABLE_NFS); > > + nr_reclaimable = global_node_page_state(NR_FILE_DIRTY); > > ... > > - gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) + > > - global_node_page_state(NR_UNSTABLE_NFS); > > + gdtc->dirty = global_node_page_state(NR_FILE_DIRTY); > > > > So if there's any negative performance impact of these changes, they're > > likely due to code alignment changes or something like that... So I don't > > think there's much to do here since optimal code alignment is highly specific > > to a particular CPU etc. > > I agree, it seems odd. > > Removing NR_UNSTABLE_NFS from enum node_stat_item would renumber all the > following value and would, I think, change NR_DIRTIED from 32 to 31. > Might that move something to a different cache line and change some > contention?
Interesting theory, it could be possible.
> That would be easy enough to test: just re-add NR_UNSTABLE_NFS.
Yeah, easy enough to test. Patch for this is attached. 0-day people, can you check whether applying this patch changes anything in your perf numbers?
> I have no experience reading will-it-scale results, but 15% does seem > like a lot.
Well, will-it-scale is a micro-benchmark that usually runs in extremely parallel loads so 15% can be caused by fairly obscure issues like different code alignment of a hot loop, slightly different cache line sharing, or so...
Honza -- Jan Kara <jack@suse.com> SUSE Labs, CR
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