From d6ceb25e8d8bccf826848c2621a50d02c0a7f4ae Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Tue, 14 Apr 2009 14:18:16 +0200 Subject: [PATCH] cfq-iosched: don't delay queue kick for a merged request "Zhang, Yanmin" reports that commit b029195dda0129b427c6e579a3bb3ae752da3a93 introduced a regression of about 50% with sequential threaded read workloads. The test case is: tiotest -k0 -k1 -k3 -f 80 -t 32 which starts 32 threads each reading a 80MB file. Twiddle the kick queue logic so that we do start IO immediately, if it appears to be a fully merged request. We can't really detect that, so just check if the request is bigger than a page or not. The assumption is that since single bio issues will first queue a single request with just one page attached and then later do merges on that, if we already have more than a page worth of data in the request, then the request is most likely good to go. Verified that this doesn't cause a regression with the test case that commit b029195dda0129b427c6e579a3bb3ae752da3a93 was fixing. It does not, we still see maximum sized requests for the queue-then-merge cases. Signed-off-by: Jens Axboe --- block/cfq-iosched.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index cba8a5d91bfa..56e9d8503cf1 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1903,10 +1903,17 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq, * Remember that we saw a request from this process, but * don't start queuing just yet. Otherwise we risk seeing lots * of tiny requests, because we disrupt the normal plugging - * and merging. + * and merging. If the request is already larger than a single + * page, let it rip immediately. For that case we assume that + * merging is already done. */ - if (cfq_cfqq_wait_request(cfqq)) + if (cfq_cfqq_wait_request(cfqq)) { + if (blk_rq_bytes(rq) > PAGE_CACHE_SIZE) { + del_timer(&cfqd->idle_slice_timer); + blk_start_queueing(cfqd->queue); + } cfq_mark_cfqq_must_dispatch(cfqq); + } } else if (cfq_should_preempt(cfqd, cfqq, rq)) { /* * not the active queue - expire current slice if it is