[Devel] Re: [patch 0/4] [RFC] Another proportional weight IO controller

[Aaron Carroll]

Fabio Checconi wrote:
>>> Fabio Checconi wrote:
>>>>   - To detect hw tagging in BFQ we consider a sample valid iff the
>>>>     number of requests that the scheduler could have dispatched (given
>>>>     by cfqd->rb_queued + cfqd->rq_in_driver, i.e., the ones still into
>>>>     the scheduler plus the ones into the driver) is higher than the
>>>>     CFQ_HW_QUEUE_MIN threshold.  This obviously caused no problems
>>>>     during testing, but the way CFQ uses now seems a little bit
>>>>     strange.
>>> BFQ's tag detection logic is broken in the same way that CFQ's used to
>>> be.  Explanation is in this patch:
>>>
>> If you look at bfq_update_hw_tag(), the logic introduced by the patch
>> you mention is still there; BFQ starts with ->hw_tag = 1, and updates it

Yes, I missed that.  So which part of CFQ's hw_tag detection is strange?

>> every 32 valid samples.  What changed WRT your patch, apart from the
>> number of samples, is that the condition for a sample to be valid is:
>>
>>   bfqd->rq_in_driver + bfqd->queued >= 5
>>
>> while in your patch it is:
>>
>>   cfqd->rq_queued > 5 || cfqd->rq_in_driver > 5
>>
>> We preferred the first one because that sum better reflects the number
>> of requests that could have been dispatched, and I don't think that this
>> is wrong.

I think it's fine too.  CFQ's condition accounts for a few rare situations,
such as the device stalling or hw_tag being updated right after a bunch of
requests are queued.  They are probably irrelevant, but can't hurt.

>> There is a problem, but it's not within the tag detection logic itself.
>> From some quick experiments, what happens is that when a process starts,
>> CFQ considers it seeky (*), BFQ doesn't.  As a side effect BFQ does not
>> always dispatch enough requests to correctly detect tagging.
>>
>> At the first seek you cannot tell if the process is going to bee seeky
>> or not, and we have chosen to consider it sequential because it improved
>> fairness in some sequential workloads (the CIC_SEEKY heuristic is used
>> also to determine the idle_window length in [bc]fq_arm_slice_timer()).
>>
>> Anyway, we're dealing with heuristics, and they tend to favor some
>> workload over other ones.  If recovering this thoughput loss is more
>> important than a transient unfairness due to short idling windows assigned
>> to sequential processes when they start, I've no problems in switching
>> the CIC_SEEKY logic to consider a process seeky when it starts.
>>
>> Thank you for testing and for pointing out this issue, we missed it
>> in our testing.
>>
>>
>> (*) to be correct, the initial classification depends on the position
>>     of the first accessed sector.
> 
> Sorry, I forgot the patch...  This seems to solve the problem with
> your workload here, does it work for you?

Yes, it works fine now :)

However, hw_tag detection (in CFQ and BFQ) is still broken in a few ways:
  * If you go from queue_depth=1 to queue_depth=large, it's possible that
    the detection logic fails.  This could happen if setting queue_depth
    to a larger value at boot, which seems a reasonable situation.
  * It depends too much on the hardware.  If you have a seekly load on a
    fast disk with a unit queue depth, idling sucks for performance (I
    imagine this is particularly bad on SSDs).  If you have any disk with
    a deep queue, not idling sucks for fairness.
I suppose CFQ's slice_resid is supposed to help here, but as far as I can
tell, it doesn't do a thing.


     -- Aaron


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