From: Josef Bacik <jbacik@xxxxxx> A basic documentation to describe the interface, statistics, and behavior of io.latency. Signed-off-by: Josef Bacik <jbacik@xxxxxx> --- Documentation/blk-iolatency.txt | 80 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 80 insertions(+) create mode 100644 Documentation/blk-iolatency.txt diff --git a/Documentation/blk-iolatency.txt b/Documentation/blk-iolatency.txt new file mode 100644 index 000000000000..9dd86f4f64b6 --- /dev/null +++ b/Documentation/blk-iolatency.txt @@ -0,0 +1,80 @@ +Block IO Latency Controller + +Overview +======== + +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. + +Interface +========= + +- 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=<integer>. This is the current queue depth for the group. + - delay=<time in microseconds>. This is the current delay per task that + does IO in this group. + - use_delay=<integer>. This is how deep into the delay we currently + are, the larger this number is the longer it'll take us to get back to + queue depth > 1. + - total_lat_avg=<time in microseconds>. The running average IO latency + for this group. Running average is generally flawed, but will give an + admistrator a general idea of the overall latency they can expect for + their workload on the given disk. + +HOWTO +===== + +The limits are only applied at the peer level in the heirarchy. This means that +in the diagram below, only groups A, B, and C will influence eachother, and +groups D and F will influence eachother. 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 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 it's +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 occuring we + limit the individual delay events to 1 second at a time. + +Once the victimized group starts meeting it's latency target again it will start +unthrottling any peer groups that were throttled previous. If the victimized +group simply stops doing IO the global counter will unthrottle appropriately. -- 2.14.3
