path: root/block/cfq-iosched.c
diff options
authorJens Axboe <>2016-11-09 12:38:14 -0700
committerJens Axboe <>2016-11-10 13:53:40 -0700
commit87760e5eef359788047d6fd54fc12eec74ce0d27 (patch)
tree0c394ea517cc093d8fe837ad5a7201d0d30c7afe /block/cfq-iosched.c
parente34cbd307477ae07c5d8a8d0bd15e65a9ddaba5c (diff)
block: hook up writeback throttling
Enable throttling of buffered writeback to make it a lot more smooth, and has way less impact on other system activity. Background writeback should be, by definition, background activity. The fact that we flush huge bundles of it at the time means that it potentially has heavy impacts on foreground workloads, which isn't ideal. We can't easily limit the sizes of writes that we do, since that would impact file system layout in the presence of delayed allocation. So just throttle back buffered writeback, unless someone is waiting for it. The algorithm for when to throttle takes its inspiration in the CoDel networking scheduling algorithm. Like CoDel, blk-wb monitors the minimum latencies of requests over a window of time. In that window of time, if the minimum latency of any request exceeds a given target, then a scale count is incremented and the queue depth is shrunk. The next monitoring window is shrunk accordingly. Unlike CoDel, if we hit a window that exhibits good behavior, then we simply increment the scale count and re-calculate the limits for that scale value. This prevents us from oscillating between a close-to-ideal value and max all the time, instead remaining in the windows where we get good behavior. Unlike CoDel, blk-wb allows the scale count to to negative. This happens if we primarily have writes going on. Unlike positive scale counts, this doesn't change the size of the monitoring window. When the heavy writers finish, blk-bw quickly snaps back to it's stable state of a zero scale count. The patch registers a sysfs entry, 'wb_lat_usec'. This sets the latency target to me met. It defaults to 2 msec for non-rotational storage, and 75 msec for rotational storage. Setting this value to '0' disables blk-wb. Generally, a user would not have to touch this setting. We don't enable WBT on devices that are managed with CFQ, and have a non-root block cgroup attached. If we have a proportional share setup on this particular disk, then the wbt throttling will interfere with that. We don't have a strong need for wbt for that case, since we will rely on CFQ doing that for us. Signed-off-by: Jens Axboe <>
Diffstat (limited to 'block/cfq-iosched.c')
1 files changed, 14 insertions, 0 deletions
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6101051..e280d08 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -16,6 +16,7 @@
#include <linux/blktrace_api.h>
#include <linux/blk-cgroup.h>
#include "blk.h"
+#include "blk-wbt.h"
* tunables
@@ -3762,9 +3763,11 @@ static void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
struct cfq_data *cfqd = cic_to_cfqd(cic);
struct cfq_queue *cfqq;
uint64_t serial_nr;
+ bool nonroot_cg;
serial_nr = bio_blkcg(bio)->css.serial_nr;
+ nonroot_cg = bio_blkcg(bio) != &blkcg_root;
@@ -3775,6 +3778,17 @@ static void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
+ * If we have a non-root cgroup, we can depend on that to
+ * do proper throttling of writes. Turn off wbt for that
+ * case.
+ */
+ if (nonroot_cg) {
+ struct request_queue *q = cfqd->queue;
+ wbt_disable(q->rq_wb);
+ }
+ /*
* Drop reference to queues. New queues will be assigned in new
* group upon arrival of fresh requests.