linux-kernel - Re: BFS vs. mainline scheduler benchmarks and measurements

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Date:	Wed, 9 Sep 2009 22:50:43 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nikos Chantziaras <realnc@...or.de>
Cc:	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements

On Wed, Sep 09 2009, Nikos Chantziaras wrote:
> On 09/09/2009 09:04 PM, Ingo Molnar wrote:
>> [...]
>> * Jens Axboe<jens.axboe@...cle.com>  wrote:
>>
>>> On Wed, Sep 09 2009, Jens Axboe wrote:
>>>  [...]
>>> BFS210 runs on the laptop (dual core intel core duo). With make -j4
>>> running, I clock the following latt -c8 'sleep 10' latencies:
>>>
>>> -rc9
>>>
>>>          Max                17895 usec
>>>          Avg                 8028 usec
>>>          Stdev               5948 usec
>>>          Stdev mean           405 usec
>>>
>>>          Max                17896 usec
>>>          Avg                 4951 usec
>>>          Stdev               6278 usec
>>>          Stdev mean           427 usec
>>>
>>>          Max                17885 usec
>>>          Avg                 5526 usec
>>>          Stdev               6819 usec
>>>          Stdev mean           464 usec
>>>
>>> -rc9 + mike
>>>
>>>          Max                 6061 usec
>>>          Avg                 3797 usec
>>>          Stdev               1726 usec
>>>          Stdev mean           117 usec
>>>
>>>          Max                 5122 usec
>>>          Avg                 3958 usec
>>>          Stdev               1697 usec
>>>          Stdev mean           115 usec
>>>
>>>          Max                 6691 usec
>>>          Avg                 2130 usec
>>>          Stdev               2165 usec
>>>          Stdev mean           147 usec
>>
>> At least in my tests these latencies were mainly due to a bug in
>> latt.c - i've attached the fixed version.
>>
>> The other reason was wakeup batching. If you do this:
>>
>>     echo 0>  /proc/sys/kernel/sched_wakeup_granularity_ns
>>
>> ... then you can switch on insta-wakeups on -tip too.
>>
>> With a dual-core box and a make -j4 background job running, on
>> latest -tip i get the following latencies:
>>
>>   $ ./latt -c8 sleep 30
>>   Entries: 656 (clients=8)
>>
>>   Averages:
>>   ------------------------------
>>   	Max	      158 usec
>> 	Avg	       12 usec
>> 	Stdev	       10 usec
>
> With your version of latt.c, I get these results with 2.6-tip vs  
> 2.6.31-rc9-bfs:
>
>
> (mainline)
> Averages:
> ------------------------------
>         Max            50 usec
>         Avg            12 usec
>         Stdev           3 usec
>
>
> (BFS)
> Averages:
> ------------------------------
>         Max           474 usec
>         Avg            11 usec
>         Stdev          16 usec
>
>
> However, the interactivity problems still remain.  Does that mean it's  
> not a latency issue?

It probably just means that latt isn't a good measure of the problem.
Which isn't really too much of a surprise.

-- 
Jens Axboe

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