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sbitmap: randomize initial alloc_hint values

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In order to get good cache behavior from a sbitmap, we want each CPU to
stick to its own cacheline(s) as much as possible. This might happen
naturally as the bitmap gets filled up and the alloc_hint values spread
out, but we really want this behavior from the start. blk-mq apparently
intended to do this, but the code to do this was never wired up. Get rid
of the dead code and make it part of the sbitmap library.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Omar Sandoval 2016-09-17 01:28:25 -07:00 • committed by Jens Axboe
parent f4a644db86
commit 98d95416db
3 changed files with 6 additions and 9 deletions
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6
lib/sbitmap.c
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@ -15,6 +15,7 @@
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
#include <linux/random.h>
#include <linux/sbitmap.h>
int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@ -211,6 +212,11 @@ int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,
return -ENOMEM;
}
if (depth && !round_robin) {
for_each_possible_cpu(i)
*per_cpu_ptr(sbq->alloc_hint, i) = prandom_u32() % depth;
}
sbq->wake_batch = sbq_calc_wake_batch(depth);
atomic_set(&sbq->wake_index, 0);