From e3ba9ae58a5599226e3976b29c8093041ae7c332 Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Thu, 25 Sep 2008 11:42:41 +0200 Subject: block: reserve some tags just for sync IO By only allowing async IO to consume 3/4 ths of the tag depth, we always have slots free to serve sync IO. This is important to avoid having writes fill the entire tag queue, thus starving reads. Original patch and idea from Linus Torvalds Signed-off-by: Jens Axboe --- block/blk-tag.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/block/blk-tag.c b/block/blk-tag.c index 8a99688eb1b1..c0d419e84ce7 100644 --- a/block/blk-tag.c +++ b/block/blk-tag.c @@ -337,6 +337,7 @@ EXPORT_SYMBOL(blk_queue_end_tag); int blk_queue_start_tag(struct request_queue *q, struct request *rq) { struct blk_queue_tag *bqt = q->queue_tags; + unsigned max_depth, offset; int tag; if (unlikely((rq->cmd_flags & REQ_QUEUED))) { @@ -350,10 +351,19 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq) /* * Protect against shared tag maps, as we may not have exclusive * access to the tag map. + * + * We reserve a few tags just for sync IO, since we don't want + * to starve sync IO on behalf of flooding async IO. */ + max_depth = bqt->max_depth; + if (rq_is_sync(rq)) + offset = 0; + else + offset = max_depth >> 2; + do { - tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth); - if (tag >= bqt->max_depth) + tag = find_next_zero_bit(bqt->tag_map, max_depth, offset); + if (tag >= max_depth) return 1; } while (test_and_set_bit_lock(tag, bqt->tag_map)); -- cgit v1.2.2