Re: Mainline kernel OLTP performance update

[Gregory Haskins <ghaskins@novell.com>]

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Steven Rostedt wrote:
> On Thu, 2009-01-15 at 11:00 -0700, Matthew Wilcox wrote:
>   
>> On Thu, Jan 15, 2009 at 09:44:42AM -0800, Andrew Morton wrote:
>>     
>>>> Me too.  Anecdotally, I haven't noticed this in my lab machines, but
>>>> what I have noticed is on someone else's laptop (a hyperthreaded atom)
>>>> that I was trying to demo powertop on was that IPI reschedule interrupts
>>>> seem to be out of control ... they were ticking over at a really high
>>>> rate and preventing the CPU from spending much time in the low C and P
>>>> states.  To me this implicates some scheduler problem since that's the
>>>> primary producer of IPI reschedules ... I think it wouldn't be a
>>>> significant extrapolation to predict that the scheduler might be the
>>>> cause of the above problem as well.
>>>>
>>>>         
>>> Good point.
>>>
>>> The context switch rate actually went down a bit.
>>>
>>> I wonder if the Intel test people have records of /proc/interrupts for
>>> the various kernel versions.
>>>       
>> I think Chinang does, but he's out of office today.  He did say in an
>> earlier reply:
>>
>>     
>>> I took a quick look at the interrupts figure between 2.6.24 and 2.6.27.
>>> i/o interuputs is slightly down in 2.6.27 (due to reduce throughput).
>>> But both NMI and reschedule interrupt increased.  Reschedule interrupts
>>> is 2x of 2.6.24.
>>>       
>> So if the reschedule interrupt is happening twice as often, and the
>> context switch rate is basically unchanged, I guess that means the
>> scheduler is doing a lot more work to get approximately the same
>> results.  And that seems like a bad thing.
>>     

I would be very interested in gathering some data in this area.  One
thing that pops to mind is to instrument the resched-ipi with
ftrace_printk() and gather a trace of this system in action.  I assume
that I wouldn't have access to this OLTP suite, so I may need a
volunteer to try this for me.  I could put together an instrumentation
patch for the testers convenience if they prefer.

Another data-point I wouldn't mind seeing is  looking at the scheduler
statistics, particularly with my sched-top utility, which you can find here:

http://rt.wiki.kernel.org/index.php/Schedtop_utility

(Note you may want to exclude the sched_info stats, as they are
inherently noisy and make it hard to see the real trends.  To do this
run it with: 'schedtop -x "sched_info"'

In the meantime, I will try similar approaches here on other non-OLTP
based workloads to see if I spy anything that looks amiss.
 
-Greg



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