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authorJens Axboe <axboe@fb.com>2014-05-09 19:36:49 +0400
committerJens Axboe <axboe@fb.com>2014-05-09 19:36:49 +0400
commit4bb659b156996f2993dc16fad71fec9ee070153c (patch)
treebaff4b0882edfc01f082b321e7924674a0fe1e90 /block/blk-mq.h
parentaf76e555e5e29e08eb8ac1f7878e23dbf0d6741f (diff)
downloadlinux-4bb659b156996f2993dc16fad71fec9ee070153c.tar.xz
blk-mq: implement new and more efficient tagging scheme
blk-mq currently uses percpu_ida for tag allocation. But that only works well if the ratio between tag space and number of CPUs is sufficiently high. For most devices and systems, that is not the case. The end result if that we either only utilize the tag space partially, or we end up attempting to fully exhaust it and run into lots of lock contention with stealing between CPUs. This is not optimal. This new tagging scheme is a hybrid bitmap allocator. It uses two tricks to both be SMP friendly and allow full exhaustion of the space: 1) We cache the last allocated (or freed) tag on a per blk-mq software context basis. This allows us to limit the space we have to search. The key element here is not caching it in the shared tag structure, otherwise we end up dirtying more shared cache lines on each allocate/free operation. 2) The tag space is split into cache line sized groups, and each context will start off randomly in that space. Even up to full utilization of the space, this divides the tag users efficiently into cache line groups, avoiding dirtying the same one both between allocators and between allocator and freeer. This scheme shows drastically better behaviour, both on small tag spaces but on large ones as well. It has been tested extensively to show better performance for all the cases blk-mq cares about. Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'block/blk-mq.h')
-rw-r--r--block/blk-mq.h4
1 files changed, 3 insertions, 1 deletions
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 1ae364ceaf8b..97cfab9c092f 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -12,6 +12,8 @@ struct blk_mq_ctx {
unsigned int cpu;
unsigned int index_hw;
+ unsigned int last_tag ____cacheline_aligned_in_smp;
+
/* incremented at dispatch time */
unsigned long rq_dispatched[2];
unsigned long rq_merged;
@@ -21,7 +23,7 @@ struct blk_mq_ctx {
struct request_queue *queue;
struct kobject kobj;
-};
+} ____cacheline_aligned_in_smp;
void __blk_mq_complete_request(struct request *rq);
void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async);