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 <[email protected]> --- 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 cc345e1..3f33d86 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; } -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
