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

Darrick J. Wong
2019-07-02T14:50:09Z

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);
> > }
> >
> > /*
> >