linux-kernel - Re: [PATCH 13/15] Documentation: add a doc for blk-iolatency

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Date:   Mon, 25 Jun 2018 15:36:18 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Josef Bacik <josef@...icpanda.com>, axboe@...nel.dk,
        linux-block@...r.kernel.org, kernel-team@...com,
        akpm@...ux-foundation.org, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-fsdevel@...r.kernel.org
Cc:     Josef Bacik <jbacik@...com>
Subject: Re: [PATCH 13/15] Documentation: add a doc for blk-iolatency

On 06/25/2018 08:12 AM, Josef Bacik wrote:
> From: Josef Bacik <jbacik@...com>
> 
> A basic documentation to describe the interface, statistics, and
> behavior of io.latency.
> 
> Signed-off-by: Josef Bacik <jbacik@...com>

Hi,
Just a few comments...

> ---
>  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

                                                           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

                      conserving;

> +target the controller doesn't do anything.  Once a group starts missing it's

                                                                           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 it's latency target again it will start

                                            its

> +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"

		                                 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