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authorPaolo Valente <paolo.valente@linaro.org>2017-12-20 14:38:33 +0300
committerJens Axboe <axboe@kernel.dk>2018-01-05 19:26:09 +0300
commit7b8fa3b900a087bc03b11329a92398fde563ba37 (patch)
treefc14295feedbd3090b76d0c5191444d0df7aac40 /block/bfq-wf2q.c
parent1be6e8a964ee9aa8d4daac523ce29e5f486dd756 (diff)
downloadlinux-7b8fa3b900a087bc03b11329a92398fde563ba37.tar.xz
block, bfq: let a queue be merged only shortly after starting I/O
In BFQ and CFQ, two processes are said to be cooperating if they do I/O in such a way that the union of their I/O requests yields a sequential I/O pattern. To get such a sequential I/O pattern out of the non-sequential pattern of each cooperating process, BFQ and CFQ merge the queues associated with these processes. In more detail, cooperating processes, and thus their associated queues, usually start, or restart, to do I/O shortly after each other. This is the case, e.g., for the I/O threads of KVM/QEMU and of the dump utility. Basing on this assumption, this commit allows a bfq_queue to be merged only during a short time interval (100ms) after it starts, or re-starts, to do I/O. This filtering provides two important benefits. First, it greatly reduces the probability that two non-cooperating processes have their queues merged by mistake, if they just happen to do I/O close to each other for a short time interval. These spurious merges cause loss of service guarantees. A low-weight bfq_queue may unjustly get more than its expected share of the throughput: if such a low-weight queue is merged with a high-weight queue, then the I/O for the low-weight queue is served as if the queue had a high weight. This may damage other high-weight queues unexpectedly. For instance, because of this issue, lxterminal occasionally took 7.5 seconds to start, instead of 6.5 seconds, when some sequential readers and writers did I/O in the background on a FUJITSU MHX2300BT HDD. The reason is that the bfq_queues associated with some of the readers or the writers were merged with the high-weight queues of some processes that had to do some urgent but little I/O. The readers then exploited the inherited high weight for all or most of their I/O, during the start-up of terminal. The filtering introduced by this commit eliminated any outlier caused by spurious queue merges in our start-up time tests. This filtering also provides a little boost of the throughput sustainable by BFQ: 3-4%, depending on the CPU. The reason is that, once a bfq_queue cannot be merged any longer, this commit makes BFQ stop updating the data needed to handle merging for the queue. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/bfq-wf2q.c')
-rw-r--r--block/bfq-wf2q.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index e495d3f9b4b0..4456eda34e48 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -835,6 +835,10 @@ void bfq_bfqq_served(struct bfq_queue *bfqq, int served)
struct bfq_entity *entity = &bfqq->entity;
struct bfq_service_tree *st;
+ if (!bfqq->service_from_backlogged)
+ bfqq->first_IO_time = jiffies;
+
+ bfqq->service_from_backlogged += served;
for_each_entity(entity) {
st = bfq_entity_service_tree(entity);