Re: [PATCH V6 00/18] blk-throttle: add .low limit

[Joseph Qi]

Hi Shaohua,

On 17/9/6 05:02, Shaohua Li wrote:
> On Thu, Aug 31, 2017 at 09:24:23AM +0200, Paolo VALENTE wrote:
>>
>>> Il giorno 15 gen 2017, alle ore 04:42, Shaohua Li <shli@xxxxxx> ha scritto:
>>>
>>> Hi,
>>>
>>> cgroup still lacks a good iocontroller. CFQ works well for hard disk, but not
>>> much for SSD. This patch set try to add a conservative limit for blk-throttle.
>>> It isn't a proportional scheduling, but can help prioritize cgroups. There are
>>> several advantages we choose blk-throttle:
>>> - blk-throttle resides early in the block stack. It works for both bio and
>>>  request based queues.
>>> - blk-throttle is light weight in general. It still takes queue lock, but it's
>>>  not hard to implement a per-cpu cache and remove the lock contention.
>>> - blk-throttle doesn't use 'idle disk' mechanism, which is used by CFQ/BFQ. The
>>>  mechanism is proved to harm performance for fast SSD.
>>>
>>> The patch set add a new io.low limit for blk-throttle. It's only for cgroup2.
>>> The existing io.max is a hard limit throttling. cgroup with a max limit never
>>> dispatch more IO than its max limit. While io.low is a best effort throttling.
>>> cgroups with 'low' limit can run above their 'low' limit at appropriate time.
>>> Specifically, if all cgroups reach their 'low' limit, all cgroups can run above
>>> their 'low' limit. If any cgroup runs under its 'low' limit, all other cgroups
>>> will run according to their 'low' limit. So the 'low' limit could act as two
>>> roles, it allows cgroups using free bandwidth and it protects cgroups from
>>> their 'low' limit.
>>>
>>> An example usage is we have a high prio cgroup with high 'low' limit and a low
>>> prio cgroup with low 'low' limit. If the high prio cgroup isn't running, the low
>>> prio can run above its 'low' limit, so we don't waste the bandwidth. When the
>>> high prio cgroup runs and is below its 'low' limit, low prio cgroup will run
>>> under its 'low' limit. This will protect high prio cgroup to get more
>>> resources.
>>>
>>
>> Hi Shaohua,
> 
> Hi,
> 
> Sorry for the late response.
>> I would like to ask you some questions, to make sure I fully
>> understand how the 'low' limit and the idle-group detection work in
>> your above scenario.  Suppose that: the drive has a random-I/O peak
>> rate of 100MB/s, the high prio group has a 'low' limit of 90 MB/s, and
>> the low prio group has a 'low' limit of 10 MB/s.  If
>> - the high prio process happens to do, say, only 5 MB/s for a given
>>   long time
>> - the low prio process constantly does greedy I/O
>> - the idle-group detection is not being used
>> then the low prio process is limited to 10 MB/s during all this time
>> interval.  And only 10% of the device bandwidth is utilized.
>>
>> To recover lost bandwidth through idle-group detection, we need to set
>> a target IO latency for the high-prio group.  The high prio group
>> should happen to be below the threshold, and thus to be detected as
>> idle, leaving the low prio group free too use all the bandwidth.
>>
>> Here are my questions:
>> 1) Is all I wrote above correct?
> 
> Yes
>> 2) In particular, maybe there are other better mechanism to saturate
>> the bandwidth in the above scenario?
> 
> Assume it's the 4) below.
>> If what I wrote above is correct:
>> 3) Doesn't fluctuation occur?  I mean: when the low prio group gets
>> full bandwidth, the latency threshold of the high prio group may be
>> overcome, causing the high prio group to not be considered idle any
>> longer, and thus the low prio group to be limited again; this in turn
>> will cause the threshold to not be overcome any longer, and so on.
> 
> That's true. We try to mitigate the fluctuation by increasing the low prio
> cgroup bandwidth graduately though.
> 
>> 4) Is there a way to compute an appropriate target latency of the high
>> prio group, if it is a generic group, for which the latency
>> requirements of the processes it contains are only partially known or
>> completely unknown?  By appropriate target latency, I mean a target
>> latency that enables the framework to fully utilize the device
>> bandwidth while the high prio group is doing less I/O than its limit.
> 
> Not sure how we can do this. The device max bandwidth varies based on request
> size and read/write ratio. We don't know when the max bandwidth is reached.
> Also I think we must consider a case that the workloads never use the full
> bandwidth of a disk, which is pretty common for SSD (at least in our
> environment).
> 
I have a question on the base latency tracking.
>From my test on SSD, write latency is much lower than read when doing
mixed read/write, but currently we only track read request and then use
it's average as base latency. In other words, we don't distinguish read
and write now. As a result, all write request's latency will always be
considered as good. So I think we have to track read and write latency
separately. Or am I missing something here?

Thanks,
Joseph

> Thanks,
> Shaohua
>