Re: [mm/writeback] 8d92890bd6: will-it-scale.per_process_ops -15.3% regression

[NeilBrown]

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?

That would be easy enough to test: just re-add NR_UNSTABLE_NFS.

I have no experience reading will-it-scale results, but 15% does seem
like a lot.

NeilBrown

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