Message-ID: <4AAA64F6.2050800@redhat.com>
Date:	Fri, 11 Sep 2009 16:55:50 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
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Subject: Re: [RFC] IO scheduler based IO controller V9

Vivek Goyal wrote:
> On Fri, Sep 11, 2009 at 10:30:40AM -0400, Vivek Goyal wrote:
>> On Fri, Sep 11, 2009 at 03:16:23PM +0200, Jerome Marchand wrote:
>>> Vivek Goyal wrote:
>>>> On Thu, Sep 10, 2009 at 04:52:27PM -0400, Vivek Goyal wrote:
>>>>> On Thu, Sep 10, 2009 at 05:18:25PM +0200, Jerome Marchand wrote:
>>>>>> Vivek Goyal wrote:
>>>>>>> Hi All,
>>>>>>>
>>>>>>> Here is the V9 of the IO controller patches generated on top of 2.6.31-rc7.
>>>>>>  
>>>>>> Hi Vivek,
>>>>>>
>>>>>> I've run some postgresql benchmarks for io-controller. Tests have been
>>>>>> made with 2.6.31-rc6 kernel, without io-controller patches (when
>>>>>> relevant) and with io-controller v8 and v9 patches.
>>>>>> I set up two instances of the TPC-H database, each running in their
>>>>>> own io-cgroup. I ran two clients to these databases and tested on each
>>>>>> that simple request:
>>>>>> $ select count(*) from LINEITEM;
>>>>>> where LINEITEM is the biggest table of TPC-H (6001215 entries,
>>>>>> 720MB). That request generates a steady stream of IOs.
>>>>>>
>>>>>> Time is measure by psql (\timing switched on). Each test is run twice
>>>>>> or more if there is any significant difference between the first two
>>>>>> runs. Before each run, the cache is flush:
>>>>>> $ echo 3 > /proc/sys/vm/drop_caches
>>>>>>
>>>>>>
>>>>>> Results with 2 groups of same io policy (BE) and same io weight (1000):
>>>>>>
>>>>>> 	w/o io-scheduler	io-scheduler v8		io-scheduler v9
>>>>>> 	first	second		first	second		first	second
>>>>>> 	DB	DB		DB	DB		DB	DB
>>>>>>
>>>>>> CFQ	48.4s	48.4s		48.2s	48.2s		48.1s	48.5s
>>>>>> Noop	138.0s	138.0s		48.3s	48.4s		48.5s	48.8s
>>>>>> AS	46.3s	47.0s		48.5s	48.7s		48.3s	48.5s
>>>>>> Deadl.	137.1s	137.1s		48.2s	48.3s		48.3s	48.5s
>>>>>>
>>>>>> As you can see, there is no significant difference for CFQ
>>>>>> scheduler.
>>>>> Thanks Jerome.  
>>>>>
>>>>>> There is big improvement for noop and deadline schedulers
>>>>>> (why is that happening?).
>>>>> I think because now related IO is in a single queue and it gets to run
>>>>> for 100ms or so (like CFQ). So previously, IO from both the instances
>>>>> will go into a single queue which should lead to more seeks as requests
>>>>> from two groups will kind of get interleaved.
>>>>>
>>>>> With io controller, both groups have separate queues so requests from
>>>>> both the data based instances will not get interleaved (This almost
>>>>> becomes like CFQ where ther are separate queues for each io context
>>>>> and for sequential reader, one io context gets to run nicely for certain
>>>>> ms based on its priority).
>>>>>
>>>>>> The performance with anticipatory scheduler
>>>>>> is a bit lower (~4%).
>>>>>>
>>>> Hi Jerome, 
>>>>
>>>> Can you also run the AS test with io controller patches and both the
>>>> database in root group (basically don't put them in to separate group). I 
>>>> suspect that this regression might come from that fact that we now have
>>>> to switch between queues and in AS we wait for request to finish from
>>>> previous queue before next queue is scheduled in and probably that is
>>>> slowing down things a bit.., just a wild guess..
>>>>
>>> Hi Vivek,
>>>
>>> I guess that's not the reason. I got 46.6s for both DB in root group with
>>> io-controller v9 patches. I also rerun the test with DB in different groups
>>> and found about the same result as above (48.3s and 48.6s).
>>>
>> Hi Jerome,
>>
>> Ok, so when both the DB's are in root group (with io-controller V9
>> patches), then you get 46.6 seconds time for both the DBs. That means there
>> is no regression in this case. In this case there is only one queue of 
>> root group and AS is running timed read/write batches on this queue.
>>
>> But when both the DBs are put in separate groups then you get 48.3 and
>> 48.6 seconds respectively and we see regression. In this case there are
>> two queues belonging to each group. Elevator layer takes care of queue
>> group queue switch and AS runs timed read/write batches on these queues.
>>
>> If it is correct, then it does not exclude the possiblity that it is queue
>> switching overhead between groups?
>>  
> 
> Does your hard drive support command queuing? May be we are driving deeper
> queue depths for reads and during queue switch we will wait for requests
> to finish from last queue to finish before next queue is scheduled in (for
> AS) and that probably will cause more delay if we are driving deeper queue
> depth.
> 
> Can you please set queue depth to "1" (/sys/block/<disk>/device/queue_depth) on
> this disk and see time consumed in two cases are same or different. I think
> setting depth to "1" will bring down overall throughput but if times are same
> in two cases, at least we will know where the delay is coming from.
> 
> Thanks
> Vivek

It looks like command queuing is supported but disabled. Queue depth is already 1
and the file /sys/block/<disk>/device/queue_depth is read-only.

Jerome
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