xfs: increase the default parallelism levels of pwork clientspwork-parallelism_2020-10-26

Increase the default parallelism level for pwork clients so that we can
take advantage of computers with a lot of CPUs and a lot of hardware.

8x raid0 spinning rust running quotacheck:
1		39s
2		29s
4		26s
8		24s
24 (nr_cpus)	24s

4x raid0 sata ssds running quotacheck:
1		12s
2		12s
4		12s
8		13s
24 (nr_cpus)	14s

4x raid0 nvme ssds running quotacheck:
1		18s
2		18s
4		19s
8		20s
20 (nr_cpus)	20s

So, mixed results...

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 4e4cf91f4f9f..f3ae327cc8f4 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -2384,3 +2384,37 @@ xfs_verify_magic16(
 		return false;
 	return dmagic == bp->b_ops->magic16[idx];
 }
+
+/* Estimate the amount of parallelism available for a given device. */
+unsigned int
+xfs_buftarg_guess_threads(
+	struct xfs_buftarg	*btp)
+{
+	int			iomin;
+	int			ioopt;
+
+	/*
+	 * The device tells us that it is non-rotational, and we take that to
+	 * mean there are no moving parts and that the device can handle all
+	 * the CPUs throwing IO requests at it.
+	 */
+	if (blk_queue_nonrot(btp->bt_bdev->bd_disk->queue))
+		return num_online_cpus();
+
+	/*
+	 * The device has a preferred and minimum IO size that suggest a RAID
+	 * setup, so infer the number of disks and assume that the parallelism
+	 * is equal to the disk count.
+	 */
+	iomin = bdev_io_min(btp->bt_bdev);
+	ioopt = bdev_io_opt(btp->bt_bdev);
+	if (iomin > 0 && ioopt > iomin)
+		return ioopt / iomin;
+
+	/*
+	 * The device did not indicate that it has any capabilities beyond that
+	 * of a rotating disk with a single drive head, so we estimate no
+	 * parallelism at all.
+	 */
+	return 1;
+}