[PATCH v3 6/6] xfs: introduce software write streams

[Kanchan Joshi <joshi.k@samsung.com>]

Even when the underlying block device does not advertise write streams,
XFS can choose do so, as that enables logical spatial isolation and
dynamic AG-set based concurrency for the standard storage, excluding
rtvolume.

Use AG count based heuristic to derive AG set size and software streams.
Larger filesystem (i.e., more AGs) get wider fanout (i.e., larger AG-set).

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
---
 fs/xfs/xfs_inode.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 2e7c61d71b48..10ffed130dce 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -45,6 +45,7 @@
 #include "xfs_inode_util.h"
 #include "xfs_metafile.h"
 
+#define XFS_MAX_USER_WRITE_STREAMS		(16)
 struct kmem_cache *xfs_inode_cache;
 
 int
@@ -52,12 +53,34 @@ xfs_inode_max_write_streams(
 	struct xfs_inode	*ip)
 {
 	struct block_device	*bdev;
+	struct xfs_mount	*mp = ip->i_mount;
+	int nr_streams;
+	xfs_agnumber_t nr_ags, ag_set_size;
 
 	bdev = xfs_inode_buftarg(ip)->bt_bdev;
 	if (!bdev)
 		return 0;
 
-	return bdev_max_write_streams(bdev);
+	nr_streams = bdev_max_write_streams(bdev);
+	if (nr_streams > 0)
+		return nr_streams;
+	if (XFS_IS_REALTIME_INODE(ip))
+		return 0;
+	/*
+	 * Enable software-only streams if hardware streams are not available.
+	 * This helps to
+	 * - improve isolation; reduce allocation interleaving.
+	 * - improve concurrency using AG-set based steering within and across streams.
+	 */
+	nr_ags = mp->m_sb.sb_agcount;
+	if (nr_ags >= 32)
+		ag_set_size = 4;
+	else if (nr_ags >= 8)
+		ag_set_size = 2;
+	else
+		ag_set_size = 1;
+	nr_streams = nr_ags / ag_set_size;
+	return min_t(uint16_t, nr_streams, XFS_MAX_USER_WRITE_STREAMS);
 }
 
 uint16_t
-- 
2.25.1