path: root/block/blk-mq-tag.h
diff options
authorJens Axboe <>2014-05-09 13:41:15 -0600
committerJens Axboe <>2014-05-09 13:41:15 -0600
commit59d13bf5f57ded658c872fa22276f75ab8f12841 (patch)
treeea3b3bf9395c01a5443c2e01874837a19d6e4c62 /block/blk-mq-tag.h
parent4bb659b156996f2993dc16fad71fec9ee070153c (diff)
blk-mq: use sparser tag layout for lower queue depth
For best performance, spreading tags over multiple cachelines makes the tagging more efficient on multicore systems. But since we have 8 * sizeof(unsigned long) tags per cacheline, we don't always get a nice spread. Attempt to spread the tags over at least 4 cachelines, using fewer number of bits per unsigned long if we have to. This improves tagging performance in setups with 32-128 tags. For higher depths, the spread is the same as before (BITS_PER_LONG tags per cacheline). Signed-off-by: Jens Axboe <>
Diffstat (limited to 'block/blk-mq-tag.h')
1 files changed, 4 insertions, 3 deletions
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 06d4a2f0f7a0..7aa9f0665489 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -11,8 +11,8 @@ struct bt_wait_state {
wait_queue_head_t wait;
} ____cacheline_aligned_in_smp;
-#define TAG_TO_INDEX(tag) ((tag) / BITS_PER_LONG)
-#define TAG_TO_BIT(tag) ((tag) & (BITS_PER_LONG - 1))
+#define TAG_TO_INDEX(bt, tag) ((tag) >> (bt)->bits_per_word)
+#define TAG_TO_BIT(bt, tag) ((tag) & ((1 << (bt)->bits_per_word) - 1))
struct blk_mq_bitmap {
unsigned long word;
@@ -22,9 +22,10 @@ struct blk_mq_bitmap {
struct blk_mq_bitmap_tags {
unsigned int depth;
unsigned int wake_cnt;
+ unsigned int bits_per_word;
- struct blk_mq_bitmap *map;
unsigned int map_nr;
+ struct blk_mq_bitmap *map;
unsigned int wake_index;
struct bt_wait_state *bs;