Message-ID: <20090908080427.GA7070@elte.hu>
Date:	Tue, 8 Sep 2009 10:04:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Pietikainen <pp@...oulu.fi>
Cc:	Michael Buesch <mb@...sch.de>, Con Kolivas <kernel@...ivas.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>, Felix Fietkau <nbd@...nwrt.org>
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements


* Pekka Pietikainen <pp@...oulu.fi> wrote:

> On Mon, Sep 07, 2009 at 10:57:01PM +0200, Ingo Molnar wrote:
> > > > Could you profile it please? Also, what's the context-switch rate?
> > > 
> > > As far as I can tell, the broadcom mips architecture does not have 
> > > profiling support. It does only have some proprietary profiling 
> > > registers that nobody wrote kernel support for, yet.
> > Well, what does 'vmstat 1' show - how many context switches are 
> > there per second on the iperf server? In theory if it's a truly 
> > saturated box, there shouldnt be many - just a single iperf task 
>
> Yay, finally something that's measurable in this thread \o/

My initial posting in this thread contains 6 separate types of 
measurements, rather extensive ones. Out of those, 4 measurements 
were latency oriented, two were throughput oriented. Plenty of data, 
plenty of results, and very good reproducability.

> Gigabit Ethernet iperf on an Atom or so might be something that 
> shows similar effects yet is debuggable. Anyone feel like taking a 
> shot?

I tried iperf on x86 and simulated saturation and no, there's no BFS 
versus mainline performance difference that i can measure - simply 
because a saturated iperf server does not schedule much - it's busy 
handling all that networking workload.

I did notice that iperf is somewhat noisy: it can easily have weird 
outliers regardless of which scheduler is used. That could be an 
effect of queueing/timing: depending on precisely what order packets 
arrive and they get queued by the networking stack, does get a 
cache-effective pathway of packets get opened - while with slightly 
different timings, that pathway closes and we get much worse 
queueing performance. I saw noise on the order of magnitude of 10%, 
so iperf has to be measured carefully before drawing conclusions.

> That beast doing iperf probably ends up making it go quite close 
> to it's limits (IO, mem bw, cpu). IIRC the routing/bridging 
> performance is something like 40Mbps (depends a lot on the model, 
> corresponds pretty well with the Mhz of the beast).
> 
> Maybe not totally unlike what make -j16 does to a 1-4 core box?

No, a single iperf session is very different from kbuild make -j16. 

Firstly, iperf server is just a single long-lived task - so we 
context-switch between that and the idle thread , [and perhaps a 
kernel thread such as ksoftirqd]. The scheduler essentially has no 
leeway what task to schedule and for how long: if there's work going 
on the iperf server task will run - if there's none, the idle task 
runs. [modulo ksoftirqd - depending on the driver model and 
dependent on precise timings.]

kbuild -j16 on the other hand is a complex hierarchy and mixture of 
thousands of short-lived and long-lived tasks. The scheduler has a 
lot of leeway to decide what to schedule and for how long.

>From a scheduler perspective the two workloads could not be any more 
different. Kbuild does test scheduler decisions in non-trivial ways 
- iperf server does not really.

	Ingo
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