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SubjectRe: [RFC PATCH] Bio Throttling support for block IO controller
On Thu, Sep 2, 2010 at 8:18 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Wed, Sep 01, 2010 at 04:07:56PM -0400, Vivek Goyal wrote:
>> On Wed, Sep 01, 2010 at 01:58:30PM -0400, Vivek Goyal wrote:
>> > Hi,
>> >
>> > Currently CFQ provides the weight based proportional division of bandwidth.
>> > People also have been looking at extending block IO controller to provide
>> > throttling/max bandwidth control.
>> >
>> > I have started to write the support for throttling in block layer on
>> > request queue so that it can be used both for higher level logical
>> > devices as well as leaf nodes. This patch is still work in progress but
>> > I wanted to post it for early feedback.
>> >
>> > Basically currently I have hooked into __make_request() function to
>> > check which cgroup bio belongs to and if it is exceeding the specified
>> > BW rate. If no, thread can continue to dispatch bio as it is otherwise
>> > bio is queued internally and dispatched later with the help of a worker
>> > thread.
>> >
>> > HOWTO
>> > =====
>> > - Mount blkio controller
>> >     mount -t cgroup -o blkio none /cgroup/blkio
>> >
>> > - Specify a bandwidth rate on particular device for root group. The format
>> >   for policy is "<major>:<minor>  <byes_per_second>".
>> >
>> >     echo "8:16  1048576" > /cgroup/blkio/blkio.read_bps_device
>> >
>> >   Above will put a limit of 1MB/second on reads happening for root group
>> >   on device having major/minor number 8:16.
>> >
>> > - Run dd to read a file and see if rate is throttled to 1MB/s or not.
>> >
>> >     # dd if=/mnt/common/zerofile of=/dev/null bs=4K count=1024 iflag=direct
>> >     1024+0 records in
>> >     1024+0 records out
>> >     4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
>> >
>> >  Limits for writes can be put using blkio.write_bps_device file.
>> >
>> > Open Issues
>> > ===========
>> > - Do we need to provide additional queue congestion semantics as we are
>> >   throttling and queuing bios at request queue and probably we don't want
>> >   a user space application to consume all the memory allocating bios
>> >   and bombarding request queue with those bios.
>> >
>> > - How to handle the current blkio cgroup stats file and two policies
>> >   in the background. If for some reason both throttling and proportional
>> >   BW policies are operating on request queue, then stats will be very
>> >   confusing.
>> >
>> >   May be we can allow activating either throttling or proportional BW
>> >   policy per request queue and we can create a /sys tunable to list and
>> >   chose between policies (something like choosing IO scheduler). The
>> >   only downside of this apporach is that user also need to be aware of
>> >   the storage hierachy and activate right policy at each node/request
>> >   queue.
>>
>> Thinking more about it. The issue of stats from proportional bandwidth
>> controller and max bandwidth controller clobbering each other can
>> probably be solved by also specifying policy name with the stat. For
>> example, currently blkio.io_serviced, looks as follows.
>>
>> # cat blkio.io_serviced
>> 253:2 Read 61
>> 253:2 Write 0
>> 253:2 Sync 61
>> 253:2 Async 0
>> 253:2 Total 61
>>
>> We can introduce one more field to specify policy for which this stats are as
>> follows.
>>
>> # cat blkio.io_serviced
>> 253:2 Read 61 throttle
>> 253:2 Write 0 throttle
>> 253:2 Sync 61 throttle
>> 253:2 Async 0 throttle
>> 253:2 Total 61        throttle
>>
>> 253:2 Read 61 proportional
>> 253:2 Write 0 proportional
>> 253:2 Sync 61   proportional
>> 253:2 Async 0   proportional
>> 253:2 Total 61  proportional
>>
>
> Option 1
> ========
> I was looking at the blkio stat code more. It seems to be key value pair
> thing. So looks like I shall have to change the format of the file and
> use second field for policy name and that will break any existing tools
> parsing these blkio cgroup files.
>
> # cat blkio.io_serviced
> 253:2 throttle Read 61
> 253:2 throttle Write 0
> 253:2 throttle Sync 61
> 253:2 throttle Async 0
> 253:2 throttle Total 61
>
> 253:2 proportional Read 61
> 253:2 proportional Write 0
> 253:2 proportional Sync 61
> 253:2 proportional Async 0
> 253:2 proportional Total 61
>
> Option 2
> ========
> Introduce policy column only for new policy.
>
> 253:2 Read 61
> 253:2 Write 0
> 253:2 Sync 61
> 253:2 Async 0
> 253:2 Total 61
>
> 253:2 throttle Read 61
> 253:2 throttle Write 0
> 253:2 throttle Sync 61
> 253:2 throttle Async 0
> 253:2 throttle Total 61
>
> Here old lines continue to represent proportional weight policy stats and
> new lines with "throttle" key word represent throttling stats.
>
> This is just like adding new fields to "stat" file. I guess it might still
> might break some script which might get stumped by new lines. But if scripts
> are not parsing all the lines and just selectively picking data then these
> should be fine.
>
> Option 3
> ========
> The other option is that I introduce new cgroup files for the new
> policy. Something like what memory cgroup has done for swap accounting
> files.
>
> blkio.throttle.io_serviced
> blkio.throttle.io_service_bytes

Vivek,
I have not looked at the rest of the patch yet. But I do not get why
stats like io_serviced and io_servived_bytes would be policy specific.
They should represent the total IO from a group serviced by the disk.
If we want to count IOs which are in a new state, we should add new
stats for that. What am I missing?

>
> That will make sure ABI is not broken but number of files per cgroup
> increase and there are already significant number of files in the group.
>
> Actually I think I should atleast rename the read and write bw files so that
> they explicitly tell these belong to throtlling poilcy.
>
> blkio.throttle.read_bps_device
> blkio.throttle.write_bps_device
>
> Any thoughts on what is the best way forward.
>
> Vivek
>
>> It will allow us following.
>>
>> - Avoid two control policies overwritting each other's stats.
>> - Allow both policies (throttle, proportional) to be operational on
>>   same request queue at the same time instead of forcing user to choose
>>   one.
>> - We don't have to introduce another /sys variable per request queue and
>>   that will make life easier in terms of configuration.
>>
>> Thoughts?
>>
>> Vivek
>
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