From a6b3f7614ca690e49e934c291f707b0c19312194 Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Fri, 28 Jun 2013 21:32:27 +0200 Subject: [PATCH] block: Reserve only one queue tag for sync IO if only 3 tags are available In case a device has three tags available we still reserve two of them for sync IO. That leaves only a single tag for async IO such as writeback from flusher thread which results in poor performance. Allow async IO to consume two tags in case queue has three tag availabe to get a decent async write performance. This patch improves streaming write performance on a machine with such disk from ~21 MB/s to ~52 MB/s. Also postmark throughput in presence of streaming writer improves from 8 to 12 transactions per second so sync IO doesn't seem to be harmed in presence of heavy async writer. Signed-off-by: Jan Kara Signed-off-by: Jens Axboe --- block/blk-tag.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/block/blk-tag.c b/block/blk-tag.c index cc345e1d8d4e..3f33d8672268 100644 --- a/block/blk-tag.c +++ b/block/blk-tag.c @@ -348,9 +348,16 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq) */ max_depth = bqt->max_depth; if (!rq_is_sync(rq) && max_depth > 1) { - max_depth -= 2; - if (!max_depth) + switch (max_depth) { + case 2: max_depth = 1; + break; + case 3: + max_depth = 2; + break; + default: + max_depth -= 2; + } if (q->in_flight[BLK_RW_ASYNC] > max_depth) return 1; }