From: Josef Bacik <email@example.com> To: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Cc: Josef Bacik <firstname.lastname@example.org> Subject: [PATCH 13/13] Documentation: add a doc for blk-iolatency Date: Tue, 5 Jun 2018 09:29:48 -0400 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> From: Josef Bacik <email@example.com> A basic documentation to describe the interface, statistics, and behavior of io.latency. Signed-off-by: Josef Bacik <firstname.lastname@example.org> --- Documentation/cgroup-v2.txt | 79 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 79 insertions(+) diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt index 74cdeaed9f7a..f6684ec99720 100644 --- a/Documentation/cgroup-v2.txt +++ b/Documentation/cgroup-v2.txt @@ -51,6 +51,9 @@ v1 is available under Documentation/cgroup-v1/. 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 @@ -1395,6 +1398,82 @@ writeback as follows. 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 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 occurring 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 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 --- -- 2.14.3
next prev parent reply other threads:[~2018-06-05 13:30 UTC|newest] Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-06-05 13:29 [PATCH 00/13][V2] Introduce io.latency io controller for cgroups Josef Bacik 2018-06-05 13:29 ` [PATCH 01/13] block: add bi_blkg to the bio " Josef Bacik 2018-06-05 13:29 ` [PATCH 02/13] block: introduce bio_issue_as_root_blkg Josef Bacik 2018-06-05 13:29 ` [PATCH 03/13] blk-cgroup: allow controllers to output their own stats Josef Bacik 2018-06-05 13:29 ` [PATCH 04/13] blk: introduce REQ_SWAP Josef Bacik 2018-06-05 13:29 ` [PATCH 05/13] swap,blkcg: issue swap io with the appropriate context Josef Bacik 2018-06-05 19:41 ` Tejun Heo 2018-06-11 14:06 ` Johannes Weiner 2018-06-05 13:29 ` [PATCH 06/13] blkcg: add generic throttling mechanism Josef Bacik 2018-06-05 20:45 ` Tejun Heo 2018-06-05 13:29 ` [PATCH 07/13] memcontrol: schedule throttling if we are congested Josef Bacik 2018-06-05 20:46 ` Tejun Heo 2018-06-11 14:08 ` Johannes Weiner 2018-06-05 13:29 ` [PATCH 08/13] blk-stat: export helpers for modifying blk_rq_stat Josef Bacik 2018-06-05 13:29 ` [PATCH 09/13] blk-rq-qos: refactor out common elements of blk-wbt Josef Bacik 2018-06-05 13:29 ` [PATCH 10/13] block: remove external dependency on wbt_flags Josef Bacik 2018-06-05 13:29 ` [PATCH 11/13] rq-qos: introduce dio_bio callback Josef Bacik 2018-06-05 13:29 ` [PATCH 12/13] block: introduce blk-iolatency io controller Josef Bacik 2018-06-05 13:29 ` Josef Bacik [this message] -- strict thread matches above, loose matches on Subject: below -- 2018-05-29 21:17 [PATCH 00/13] Introdue io.latency io controller for cgroups Josef Bacik 2018-05-29 21:17 ` [PATCH 13/13] Documentation: add a doc for blk-iolatency Josef Bacik 2018-05-30 16:44 ` Tejun Heo 2018-05-30 18:32 ` Randy Dunlap
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