Date:	Mon, 21 May 2007 05:08:37 -0700
From:	William Lee Irwin III <wli@...omorphy.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch] CFS scheduler, -v12

* William Lee Irwin III <wli@...omorphy.com> wrote:
>> cfs should probably consider aggregate lag as opposed to aggregate 
>> weighted load. Mainline's convergence to proper CPU bandwidth 
>> distributions on SMP (e.g. N+1 tasks of equal nice on N cpus) is 
>> incredibly slow and probably also fragile in the presence of arrivals 
>> and departures partly because of this. [...]

On Mon, May 21, 2007 at 10:57:03AM +0200, Ingo Molnar wrote:
> hm, have you actually tested CFS before coming to this conclusion?
> CFS is fair even on SMP. Consider for example the worst-case 

No. It's mostly a response to Dmitry's suggestion. I've done all of
the benchmark/testcase-writing on mainline.


On Mon, May 21, 2007 at 10:57:03AM +0200, Ingo Molnar wrote:
> 3-tasks-on-2-CPUs workload on a 2-CPU box:
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  2658 mingo     20   0  1580  248  200 R   67  0.0   0:56.30 loop
>  2656 mingo     20   0  1580  252  200 R   66  0.0   0:55.55 loop
>  2657 mingo     20   0  1576  248  200 R   66  0.0   0:55.24 loop

This looks like you've repaired the slow convergence issue mainline
has by other means.


On Mon, May 21, 2007 at 10:57:03AM +0200, Ingo Molnar wrote:
> 66% of CPU time for each task. The 'TIME+' column shows a 2% spread 
> between the slowest and the fastest loop after just 1 minute of runtime 
> (and the spread gets narrower with time). Mainline does a 50% / 50% / 
> 100% split:
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  3121 mingo     25   0  1584  252  204 R  100  0.0   0:13.11 loop
>  3120 mingo     25   0  1584  256  204 R   50  0.0   0:06.68 loop
>  3119 mingo     25   0  1584  252  204 R   50  0.0   0:06.64 loop
> and i fixed that in CFS.

I found that mainline actually converges to the evenly-split shares of
CPU bandwidth, albeit incredibly slowly. Something like an hour is needed.


On Mon, May 21, 2007 at 10:57:03AM +0200, Ingo Molnar wrote:
> or consider a sleepy workload like massive_intr, 3-tasks-on-2-CPUs:
>   europe:~> head -1 /proc/interrupts
>              CPU0       CPU1
>   europe:~> ./massive_intr 3 10
>   002623  00000722
>   002621  00000720
>   002622  00000721
> Or a 5-tasks-on-2-CPS workload:
>   europe:~> ./massive_intr 5 50
>   002649  00002519
>   002653  00002492
>   002651  00002478
>   002652  00002510
>   002650  00002478
> that's around 1% of spread.
> load-balancing is a performance vs. fairness tradeoff so we wont be able 
> to make it precisely fair because that's hideously expensive on SMP 
> (barring someone showing a working patch of course) - but in CFS i got 
> quite close to having it very fair in practice.

This is close enough to Libenzi's load generator to mean this particular
issue is almost certainly fixed.


* William Lee Irwin III <wli@...omorphy.com> wrote:
>> [...] Tong Li's DWRR repairs the deficit in mainline by synchronizing 
>> epochs or otherwise bounding epoch dispersion. This doesn't directly 
>> translate to cfs. In cfs cpu should probably try to figure out if its 
>> aggregate lag (e.g. via minimax) is above or below average, and push 
>> to or pull from the other half accordingly.

On Mon, May 21, 2007 at 10:57:03AM +0200, Ingo Molnar wrote:
> i'd first like to see a demonstration of a problem to solve, before 
> thinking about more complex solutions ;-)

I have other, more difficult to pass testcases. I'm giving up on ipopt
for the quadratic program associated with the \ell^\infty norm and just
pushing out the least squares solution since LAPACK is standard enough
for most people to have or easily obtain.

A quick and dirty approximation is to run one task at each nice level in
a range of nice levels and see if the proportions of CPU bandwidth come
out the same on SMP as UP and how quickly they converge. The testcase
is more comprehensive than that, but it's easy enough of a check to see
if there are any issues in this area.


-- wli
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