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authorPaolo Valente <paolo.valente@linaro.org>2018-01-13 12:05:18 +0100
committerJens Axboe <axboe@kernel.dk>2018-01-18 08:21:37 -0700
commit8a8747dc01cee6f92a52c03ba686e9f60cb46c87 (patch)
treef67e62156179ca27ac54ca22efdd9d18dfdb28f6 /block/bfq-wf2q.c
parenta52a69ea89dc12e6f4572f554940789c1ab23c7a (diff)
downloadlinux-8a8747dc01cee6f92a52c03ba686e9f60cb46c87.tar.bz2
block, bfq: limit sectors served with interactive weight raising
To maximise responsiveness, BFQ raises the weight, and performs device idling, for bfq_queues associated with processes deemed as interactive. In particular, weight raising has a maximum duration, equal to the time needed to start a large application. If a weight-raised process goes on doing I/O beyond this maximum duration, it loses weight-raising. This mechanism is evidently vulnerable to the following false positives: I/O-bound applications that will go on doing I/O for much longer than the duration of weight-raising. These applications have basically no benefit from being weight-raised at the beginning of their I/O. On the opposite end, while being weight-raised, these applications a) unjustly steal throughput to applications that may truly need low latency; b) make BFQ uselessly perform device idling; device idling results in loss of device throughput with most flash-based storage, and may increase latencies when used purposelessly. This commit adds a countermeasure to reduce both the above problems. To introduce this countermeasure, we provide the following extra piece of information (full details in the comments added by this commit). During the start-up of the large application used as a reference to set the duration of weight-raising, involved processes transfer at most ~110K sectors each. Accordingly, a process initially deemed as interactive has no right to be weight-raised any longer, once transferred 110K sectors or more. Basing on this consideration, this commit early-ends weight-raising for a bfq_queue if the latter happens to have received an amount of service at least equal to 110K sectors (actually, a little bit more, to keep a safety margin). I/O-bound applications that reach a high throughput, such as file copy, get to this threshold much before the allowed weight-raising period finishes. Thus this early ending of weight-raising reduces the amount of time during which these applications cause the problems described above. Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/bfq-wf2q.c')
-rw-r--r--block/bfq-wf2q.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index 4456eda34e48..4498c43245e2 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -838,6 +838,9 @@ void bfq_bfqq_served(struct bfq_queue *bfqq, int served)
if (!bfqq->service_from_backlogged)
bfqq->first_IO_time = jiffies;
+ if (bfqq->wr_coeff > 1)
+ bfqq->service_from_wr += served;
+
bfqq->service_from_backlogged += served;
for_each_entity(entity) {
st = bfq_entity_service_tree(entity);