diff options
authorJens Axboe <>2009-04-14 14:18:16 +0200
committerJens Axboe <>2009-04-15 08:28:12 +0200
commitd6ceb25e8d8bccf826848c2621a50d02c0a7f4ae (patch)
parent053c525fcf976810f023d96472f414c0d5e6339b (diff)
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 <>
1 files 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);
+ }
+ }
} else if (cfq_should_preempt(cfqd, cfqq, rq)) {
* not the active queue - expire current slice if it is