linux-kernel - Re: [PATCH 2/2] fs: generic_file_buffered_read() now uses find_get_pages

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Date:   Tue, 9 Jun 2020 21:46:29 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs: generic_file_buffered_read() now uses
 find_get_pages_contig

On Tue, Jun 09, 2020 at 06:38:08PM -0700, Matthew Wilcox wrote:
> On Tue, Jun 09, 2020 at 08:10:36PM -0400, Kent Overstreet wrote:
> > Convert generic_file_buffered_read() to get pages to read from in
> > batches, and then copy data to userspace from many pages at once - in
> > particular, we now don't touch any cachelines that might be contended
> > while we're in the loop to copy data to userspace.
> > 
> > This is is a performance improvement on workloads that do buffered reads
> > with large blocksizes, and a very large performance improvement if that
> > file is also being accessed concurrently by different threads.
> 
> Hey, you're stealing my performance improvements!

:)

> Granted, I haven't got to doing performance optimisations (certainly
> not in this function), but this is one of the places where THP in the
> page cache will have a useful performance improvement.
> 
> I'm not opposed to putting this in, but I may back it out as part of
> the THP work because the THPs will get the same performance improvements
> that you're seeing here with less code.

I'm an _enthusiastic_ supporter of the THP stuff (as you know), but my feeling
is that it's going to be a long time before hugepages are everywhere - and I
think even with the pagevec stuff generic_file_buffered_read() is somewhat
easier to read and deal with after this series than before it.

Though I could see the pagevec stuff making hugepage support a pain, so there is
that. Eh.