Re: [PATCH 07/15] xfs: calculate inode walk prefetch more carefully

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Jul 02, 2019 at 10:24:03AM -0400, Brian Foster wrote:
> On Wed, Jun 26, 2019 at 01:44:40PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > The existing inode walk prefetch is based on the old bulkstat code,
> > which simply allocated 4 pages worth of memory and prefetched that many
> > inobt records, regardless of however many inodes the caller requested.
> > 65536 inodes is a lot to prefetch (~32M on x64, ~512M on arm64) so let's
> > scale things down a little more intelligently based on the number of
> > inodes requested, etc.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> 
> A few nits..
> 
> >  fs/xfs/xfs_iwalk.c |   46 ++++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 44 insertions(+), 2 deletions(-)
> > 
> > 
> > diff --git a/fs/xfs/xfs_iwalk.c b/fs/xfs/xfs_iwalk.c
> > index 304c41e6ed1d..3e67d7702e16 100644
> > --- a/fs/xfs/xfs_iwalk.c
> > +++ b/fs/xfs/xfs_iwalk.c
> > @@ -333,16 +333,58 @@ xfs_iwalk_ag(
> >  	return error;
> >  }
> >  
> > +/*
> > + * We experimentally determined that the reduction in ioctl call overhead
> > + * diminishes when userspace asks for more than 2048 inodes, so we'll cap
> > + * prefetch at this point.
> > + */
> > +#define MAX_IWALK_PREFETCH	(2048U)
> > +
> 
> Something like IWALK_MAX_INODE_PREFETCH is a bit more clear IMO.

<nod>

> >  /*
> >   * Given the number of inodes to prefetch, set the number of inobt records that
> >   * we cache in memory, which controls the number of inodes we try to read
> > - * ahead.
> > + * ahead.  Set the maximum if @inode_records == 0.
> >   */
> >  static inline unsigned int
> >  xfs_iwalk_prefetch(
> >  	unsigned int		inode_records)
> 
> Perhaps this should be called 'inodes' since the function converts this
> value to inode records?

ok, I see how that could be a little confusing.

> >  {
> > -	return PAGE_SIZE * 4 / sizeof(struct xfs_inobt_rec_incore);
> > +	unsigned int		inobt_records;
> > +
> > +	/*
> > +	 * If the caller didn't tell us the number of inodes they wanted,
> > +	 * assume the maximum prefetch possible for best performance.
> > +	 * Otherwise, cap prefetch at that maximum so that we don't start an
> > +	 * absurd amount of prefetch.
> > +	 */
> > +	if (inode_records == 0)
> > +		inode_records = MAX_IWALK_PREFETCH;
> > +	inode_records = min(inode_records, MAX_IWALK_PREFETCH);
> > +
> > +	/* Round the inode count up to a full chunk. */
> > +	inode_records = round_up(inode_records, XFS_INODES_PER_CHUNK);
> > +
> > +	/*
> > +	 * In order to convert the number of inodes to prefetch into an
> > +	 * estimate of the number of inobt records to cache, we require a
> > +	 * conversion factor that reflects our expectations of the average
> > +	 * loading factor of an inode chunk.  Based on data gathered, most
> > +	 * (but not all) filesystems manage to keep the inode chunks totally
> > +	 * full, so we'll underestimate slightly so that our readahead will
> > +	 * still deliver the performance we want on aging filesystems:
> > +	 *
> > +	 * inobt = inodes / (INODES_PER_CHUNK * (4 / 5));
> > +	 *
> > +	 * The funny math is to avoid division.
> > +	 */
> 
> The last bit of this comment is unclear. What do you mean by "avoid
> division?"

"..to avoid 64-bit integer division."

> With those nits fixed up:
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> 
> > +	inobt_records = (inode_records * 5) / (4 * XFS_INODES_PER_CHUNK);
> > +
> > +	/*
> > +	 * Allocate enough space to prefetch at least two inobt records so that
> > +	 * we can cache both the record where the iwalk started and the next
> > +	 * record.  This simplifies the AG inode walk loop setup code.
> > +	 */
> > +	return max(inobt_records, 2U);
> >  }
> >  
> >  /*
> >