path: root/Documentation/block/queue-sysfs.txt
diff options
authorJens Axboe <>2016-04-26 09:14:27 -0600
committerJens Axboe <>2016-04-26 09:51:21 -0600
commit6edfbb2f08df9cc3bdb65c2f06fce94a39ab6236 (patch)
tree801df7406e2066b0e64b809d589df920dca7e429 /Documentation/block/queue-sysfs.txt
parent3cd16ff982aafeb88a259d3846c8113feb4bbf28 (diff)
writeback: throttle buffered writebackwb-buf-throttle-v5
Test patch that throttles 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. The patch registers two sysfs entries. The first one, 'wb_window_usec', defines the window of monitoring. The second one, 'wb_lat_usec', sets the latency target for the window. 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 these settings. Signed-off-by: Jens Axboe <>
Diffstat (limited to 'Documentation/block/queue-sysfs.txt')
1 files changed, 13 insertions, 0 deletions
diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
index dce25d848d92..9bc990abef4d 100644
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -151,5 +151,18 @@ device state. This means that it might not be safe to toggle the
setting from "write back" to "write through", since that will also
eliminate cache flushes issued by the kernel.
+wb_lat_usec (RW)
+If the device is registered for writeback throttling, then this file shows
+the target minimum read latency. If this latency is exceeded in a given
+window of time (see wb_window_usec), then the writeback throttling will start
+scaling back writes.
+wb_window_usec (RW)
+If the device is registered for writeback throttling, then this file shows
+the value of the monitoring window in which we'll look at the target
+latency. See wb_lat_usec.
Jens Axboe <>, February 2009