Re: [RFC] Bypass filesystems for reading cached pages

[Dave Chinner]

On Mon, Jun 22, 2020 at 04:35:05PM +0200, Andreas Gruenbacher wrote:
> On Mon, Jun 22, 2020 at 2:32 AM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > On Fri, Jun 19, 2020 at 08:50:36AM -0700, Matthew Wilcox wrote:
> > >
> > > This patch lifts the IOCB_CACHED idea expressed by Andreas to the VFS.
> > > The advantage of this patch is that we can avoid taking any filesystem
> > > lock, as long as the pages being accessed are in the cache (and we don't
> > > need to readahead any pages into the cache).  We also avoid an indirect
> > > function call in these cases.
> >
> > What does this micro-optimisation actually gain us except for more
> > complexity in the IO path?
> >
> > i.e. if a filesystem lock has such massive overhead that it slows
> > down the cached readahead path in production workloads, then that's
> > something the filesystem needs to address, not unconditionally
> > bypass the filesystem before the IO gets anywhere near it.
> 
> I'm fine with not moving that functionality into the VFS. The problem
> I have in gfs2 is that taking glocks is really expensive. Part of that
> overhead is accidental, but we definitely won't be able to fix it in
> the short term. So something like the IOCB_CACHED flag that prevents
> generic_file_read_iter from issuing readahead I/O would save the day
> for us. Does that idea stand a chance?

I have no problem with a "NOREADAHEAD" flag being passed to
generic_file_read_iter(). It's not a "already cached" flag though,
it's a "don't start any IO" directive, just like the NOWAIT flag is
a "don't block on locks or IO in progress" directive and not an
"already cached" flag. Readahead is something we should be doing,
unless a filesystem has a very good reason not to, such as the gfs2
locking case here...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx