Re: [PATCH 0/7 v1] Speed up page cache truncation

[Jan Kara <jack@suse.cz>]

On Tue 10-10-17 10:25:13, Andi Kleen wrote:
> Jan Kara <jack@suse.cz> writes:
> 
> > when rebasing our enterprise distro to a newer kernel (from 4.4 to 4.12) we
> > have noticed a regression in bonnie++ benchmark when deleting files.
> > Eventually we have tracked this down to a fact that page cache truncation got
> > slower by about 10%. There were both gains and losses in the above interval of
> > kernels but we have been able to identify that commit 83929372f629 "filemap:
> > prepare find and delete operations for huge pages" caused about 10% regression
> > on its own.
> 
> It's odd that just checking if some pages are huge should be that
> expensive, but ok ..

Yeah, I was surprised as well but profiles were pretty clear on this - part
of the slowdown was caused by loads of page->_compound_head (PageTail()
and page_compound() use that) which we previously didn't have to load at
all, part was in hpage_nr_pages() function and its use.

> > Patch 1 is an easy speedup of cancel_dirty_page(). Patches 2-6 refactor page
> > cache truncation code so that it is easier to batch radix tree operations.
> > Patch 7 implements batching of deletes from the radix tree which more than
> > makes up for the original regression.
> >
> > What do people think about this series?
> 
> Batching locks is always a good idea. You'll likely see far more benefits
> under lock contention on larger systems.
> 
> From a quick read it looks good to me.
> 
> Reviewed-by: Andi Kleen <ak@linux.intel.com>

Thanks for having a look!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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