Re: interactive task starvation

[Willy Tarreau <willy@w.ods.org>]

On Tue, Mar 21, 2006 at 07:39:11PM +0100, Rafael J. Wysocki wrote:
> On Tuesday 21 March 2006 15:39, Willy Tarreau wrote:
> > On Wed, Mar 22, 2006 at 01:19:49AM +1100, Con Kolivas wrote:
> > > On Wednesday 22 March 2006 01:17, Mike Galbraith wrote:
> > > > On Wed, 2006-03-22 at 00:53 +1100, Con Kolivas wrote:
> > > > > The yardstick for changes is now the speed of 'ls' scrolling in the
> > > > > console. Where exactly are those extra cycles going I wonder? Do you
> > > > > think the scheduler somehow makes the cpu idle doing nothing in that
> > > > > timespace? Clearly that's not true, and userspace is making something
> > > > > spin unnecessarily, but we're gonna fix that by modifying the
> > > > > scheduler.... sigh
> > > >
> > > > *Blink*
> > > >
> > > > Are you having a bad hair day??
> > > 
> > > My hair is approximately 3mm long so it's kinda hard for that to happen. 
> > > 
> > > What you're fixing with unfairness is worth pursuing. The 'ls' issue just 
> > > blows my mind though for reasons I've just said. Where are the magic cycles 
> > > going when nothing else is running that make it take ten times longer?
> > 
> > Con, those cycles are not "magic", if you look at the numbers, the time is
> > not spent in the process itself. From what has been observed since the
> > beginning, it is spent :
> >   - in other processes which are starvating the CPU (eg: X11 when xterm
> >     scrolls)
> >   - in context switches when you have a pipe somewhere and the CPU is
> >     bouncing between tasks.
> > 
> > Concerning your angriness about me being OK with (0,0) and still
> > asking for tunables, it's precisely because I know that *my* workload
> > is not everyone else's, and I don't want to conclude too quickly that
> > there are only two types of workloads.
> 
> Well, perhaps we can assume there are only two types of workloads and
> wait for a test case that will show the assumption is wrong?

It would certainly fit most usages, but as soon as we find another group
of users complaining, we will add another sysctl just for them ? Perhaps
we could just resume the two current sysctls into one called
"interactivity_boost" with a value between 0 and 100, with the ability
for any user to increase or decrease it easily ? Mainline would be
pre-configured with something reasonable, like what Mike proposed as
default values for example, and server admins would only set it to
zero while desktop-intensive users could increase it a bit if they like
to.

> > Maybe you're right, maybe you're wrong. At least you're right for as long
> > as no other workload has been identified. But thinking like this is like
> > some time ago when we thought that "if it runs XMMS without skipping,
> > it'll be OK for everyone".
> 
> However, we should not try to anticipate every possible kind of workload
> IMHO.

I generally agree on this, except that we got caught once in this area for
this exact reason.

> Greetings,
> Rafael

Regards,
Willy