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

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: darrick.wong@oracle.com
Cc: linux-xfs@vger.kernel.org, bfoster@redhat.com
Subject: [PATCH 07/15] xfs: calculate inode walk prefetch more carefully
Date: Wed, 26 Jun 2019 13:44:40 -0700	[thread overview]
Message-ID: <156158188075.495087.14228436478786857410.stgit@magnolia> (raw)
In-Reply-To: <156158183697.495087.5371839759804528321.stgit@magnolia>

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>
---
 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)
+
 /*
  * 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)
 {
-	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.
+	 */
+	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);
 }
 
 /*
