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xfs: calculate inode walk prefetch more carefully 

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>
Reviewed-by: Brian Foster <bfoster@redhat.com>
This commit is contained in:
Darrick J. Wong 2019-07-02 09:39:40 -07:00
parent 2810bd6840
commit 938c710d99
1 changed files with 45 additions and 3 deletions
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48
fs/xfs/xfs_iwalk.c
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@ -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 IWALK_MAX_INODE_PREFETCH (2048U)
/*
* 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 @inodes == 0.
*/
static inline unsigned int
xfs_iwalk_prefetch(
unsigned int inode_records)
unsigned int inodes)
{
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 (inodes == 0)
inodes = IWALK_MAX_INODE_PREFETCH;
inodes = min(inodes, IWALK_MAX_INODE_PREFETCH);
/* Round the inode count up to a full chunk. */
inodes = round_up(inodes, 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 integer division.
*/
inobt_records = (inodes * 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);
}
/*