Re: [REPORT] cfs-v4 vs sd-0.44

[Willy Tarreau <w@1wt.eu>]

On Sat, Apr 21, 2007 at 05:46:14PM +0200, Ingo Molnar wrote:
> 
> * Willy Tarreau <w@1wt.eu> wrote:
> 
> > I promised to perform some tests on your code. I'm short in time right 
> > now, but I observed behaviours that should be commented on.
> 
> thanks for the feedback!
> 
> > 3) CFS-v4
> > 
> >   Feels even better, mouse movements are very smooth even under high 
> >   load. I noticed that X gets reniced to -19 with this scheduler. I've 
> >   not looked at the code yet but this looked suspicious to me. I've 
> >   reniced it to 0 and it did not change any behaviour. Still very 
> >   good. The 64 ocbench share equal CPU time and show exact same 
> >   progress after 2000 iterations. The CPU load is more smoothly spread 
> >   according to vmstat, and there's no idle (see below). BUT I now 
> >   think it was wrong to let new processes start with no timeslice at 
> >   all, because it can take tens of seconds to start a new process when 
> >   only 64 ocbench are there. [...]
> 
> ok, i'll modify that portion and add back the 50%/50% parent/child CPU 
> time sharing approach again. (which CFS had in -v1) That should not 
> change the rest of your test and should improve the task startup 
> characteristics.

If you remember, with 50/50, I noticed some difficulties to fork many
processes. I think that during a fork(), the parent has a higher probability
of forking other processes than the child. So at least, we should use
something like 67/33 or 75/25 for parent/child.

There are many shell-scripts out there doing a lot of fork(), and it should
be reasonable to let them keep some CPU to continue to work.

Also, I believe that (in shells), most forked processes do not even consume
a full timeslice (eg: $(uname -n) is very fast). This means that assigning
them with a shorter one will not hurt them while preserving the shell's
performance against CPU hogs.

Willy