Re: [PATCH 04/26] xfs: Improve metadata buffer reclaim accountability

Brian Foster
2019-10-12T12:06:01Z

On Sat, Oct 12, 2019 at 10:13:23AM +1100, Dave Chinner wrote:
> On Fri, Oct 11, 2019 at 08:39:39AM -0400, Brian Foster wrote:
> > On Wed, Oct 09, 2019 at 02:21:02PM +1100, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > >
> > > The buffer cache shrinker frees more than just the xfs_buf slab
> > > objects - it also frees the pages attached to the buffers. Make sure
> > > the memory reclaim code accounts for this memory being freed
> > > correctly, similar to how the inode shrinker accounts for pages
> > > freed from the page cache due to mapping invalidation.
> > >
> > > We also need to make sure that the mm subsystem knows these are
> > > reclaimable objects. We provide the memory reclaim subsystem with a
> > > a shrinker to reclaim xfs_bufs, so we should really mark the slab
> > > that way.
> > >
> > > We also have a lot of xfs_bufs in a busy system, spread them around
> > > like we do inodes.
> > >
> > > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > > ---
> >
> > Seems reasonable, but for inodes we also spread the ili zone. Should we
> > not be consistent with bli's as well?
>
> bli's are reclaimed when the buffer is cleaned. ili's live for the
> live of the inode in cache. Hence bli's are short term allocations
> (much shorter than xfs_bufs they attach to) and are reclaimed much
> faster than inodes and their ilis. There's also a lot less blis than
> ili's, so the spread of their footprint across memory nodes doesn't
> matter that much. Local access for the memcpy during formatting is
> probably more important than spreading the memory usage of them
> these days, anyway.
>

Yes, the buffer/inode lifecycle difference is why why I presume bli
zones are not ZONE_RECLAIM like ili zones. This doesn't tell me anything
about why buffers should be spread around as such and buffer log items
not, though..

Brian

> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com