block: bump max plugged deferred size from 16 to 32

Particularly for NVMe with efficient deferred submission for many
requests, there are nice benefits to be seen by bumping the default max
plug count from 16 to 32. This is especially true for virtualized setups,
where the submit part is more expensive. But can be noticed even on
native hardware.

Reduce the multiple queue factor from 4 to 2, since we're changing the
default size.

While changing it, move the defines into the block layer private header.
These aren't values that anyone outside of the block layer uses, or
should use.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 6 insertions, 0 deletions

diff --git a/block/blk.h b/block/blk.h
index 7aaccbc26db4..019c50a140ba 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -225,6 +225,12 @@ void blk_account_io_start(struct request *req);
 void blk_account_io_done(struct request *req, u64 now);
 
 /*
+ * Plug flush limits
+ */
+#define BLK_MAX_REQUEST_COUNT	32
+#define BLK_PLUG_FLUSH_SIZE	(128 * 1024)
+
+/*
  * Internal elevator interface
  */
 #define ELV_ON_HASH(rq) ((rq)->rq_flags & RQF_HASHED)