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SubjectRe: [PATCH 13/14] Documentation: add a doc for blk-iolatency
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On 07/03/18 08:15, Josef Bacik wrote:
> From: Josef Bacik <jbacik@fb.com>
>
> A basic documentation to describe the interface, statistics, and
> behavior of io.latency.
>
> Signed-off-by: Josef Bacik <jbacik@fb.com>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 79 +++++++++++++++++++++++++++++++++
> 1 file changed, 79 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 8a2c52d5c53b..569ce27b85e5 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -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
> @@ -1446,6 +1449,82 @@ 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 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"

(repeat comment:) 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
> ---
>
>


--
~Randy

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