5-3. IO
5-3-1. IO Interface Files
5-3-2. Writeback
+ 5-3-3. IO Latency
+ 5-3-3-1. How IO Latency Throttling Works
+ 5-3-3-2. IO Latency Interface Files
5-4. PID
5-4-1. PID Interface Files
5-5. Device
vm.dirty[_background]_ratio.
+IO Latency
+~~~~~~~~~~
+
+This is a cgroup v2 controller for IO workload protection. You provide a group
+with a latency target, and if the average latency exceeds that target the
+controller will throttle any peers that have a lower latency target than the
+protected workload.
+
+The limits are only applied at the peer level in the hierarchy. This means that
+in the diagram below, only groups A, B, and C will influence each other, and
+groups D and F will influence each other. Group G will influence nobody.
+
+ [root]
+ / | \
+ A B C
+ / \ |
+ D F G
+
+
+So the ideal way to configure this is to set io.latency in groups A, B, and C.
+Generally you do not want to set a value lower than the latency your device
+supports. Experiment to find the value that works best for your workload.
+Start at higher than the expected latency for your device and watch the
+total_lat_avg value in io.stat for your workload group to get an idea of the
+latency you see during normal operation. Use this value as a basis for your
+real setting, setting at 10-15% higher than the value in io.stat.
+Experimentation is key here because total_lat_avg is a running total, so is the
+"statistics" portion of "lies, damned lies, and statistics."
+
+How IO Latency Throttling Works
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+io.latency is work conserving; so as long as everybody is meeting their latency
+target the controller doesn't do anything. Once a group starts missing its
+target it begins throttling any peer group that has a higher target than itself.
+This throttling takes 2 forms:
+
+- Queue depth throttling. This is the number of outstanding IO's a group is
+ allowed to have. We will clamp down relatively quickly, starting at no limit
+ and going all the way down to 1 IO at a time.
+
+- Artificial delay induction. There are certain types of IO that cannot be
+ throttled without possibly adversely affecting higher priority groups. This
+ includes swapping and metadata IO. These types of IO are allowed to occur
+ normally, however they are "charged" to the originating group. If the
+ originating group is being throttled you will see the use_delay and delay
+ fields in io.stat increase. The delay value is how many microseconds that are
+ being added to any process that runs in this group. Because this number can
+ grow quite large if there is a lot of swapping or metadata IO occurring we
+ limit the individual delay events to 1 second at a time.
+
+Once the victimized group starts meeting its latency target again it will start
+unthrottling any peer groups that were throttled previously. If the victimized
+group simply stops doing IO the global counter will unthrottle appropriately.
+
+IO Latency Interface Files
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ io.latency
+ This takes a similar format as the other controllers.
+
+ "MAJOR:MINOR target=<target time in microseconds"
+
+ io.stat
+ If the controller is enabled you will see extra stats in io.stat in
+ addition to the normal ones.
+
+ depth
+ This is the current queue depth for the group.
+
+ avg_lat
+ The running average IO latency for this group in microseconds.
+ Running average is generally flawed, but will give an
+ administrator a general idea of the overall latency they can
+ expect for their workload on the given disk.
+
PID
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