Re: [2.4.17/18pre] VM and swap - it's really unusable

Dieter Nützel
2002-01-09T00:15:09Z

On Wednesday, 9. January 2002 00:02, Luigi Genoni wrote:
> On Tue, 8 Jan 2002, Daniel Phillips wrote:
> > On January 8, 2002 04:29 pm, Andrea Arcangeli wrote:
[-]
> > > I also don't want to devaluate the preemptive kernel approch (the mean
> > > latency it can reach is lower than the one of the lowlat kernel,
> > > however I personally care only about worst case latency and this is why
> > > I don't feel the need of -preempt),
> >
> > This is exactly the case that -preempt handles well. On the other hand,
> > trying to show that scheduling hacks satisfy any given latency bound is
> > equivalent to solving the halting problem.
> >
> > I thought you had done some real time work?
> >
> > > but I just wanted to make clear that the
> > > idea that is floating around that preemptive kernel is all goodness is
> > > very far from reality, you get very low mean latency but at a price.
> >
> > A price lots of people are willing to pay
>
> Probably sometimes they are not making a good business. In the reality
> preempt is good in many scenarios, as I said, and I agree that for
> desktops, and dedicated servers where just one application runs, and
> probably the CPU is idle the most of the time,

OK, good. You are much at the same line than I am.

Should we starting not only to differentiate between UP and SMP systems but
allthought between desktop and (big) servers?
I remember one saying. "Think, this patch is worth only for ~0.05% of the
Linux users..." (He meant the multi SMP system users.)

Allmost 99.95% of the Linux users running desktops and I am somewhat tiered
of saying, "sorry, Linux is under development..."
Look at the imprint of the famous German ct magazine (they are not even known
as Linux bashers...;-). It shows little penguins falling like domino stones
(starting with 2.4.17).

Let me rephrase it:
I appreciate all your great work and I know "only" some (little) internals of
it but we should do some interactivity improvements for the 2.4 kernel, too.
I know what it's worth Andrew's (lowlatency patch) and Robert's (George
Anzinger's) preempt patch. In short the system (bigger desktop) flies.

The holly grail would be a combination of preempt+lock-break plus lowlatency
and Ingo's O(1) scheduler.

My main focus lies on 3D graphics not kernel and I use KDE (yes, a little
luxury:-) 'cause KDE is C++ and most visualization systems are c and later
c++.

Without the above patches even my 1 GHz Athlon II, 640 MB, feels sluggish.
But I don't forget to think about throughput which is even usefull for
"heavy" compiler runs...

> indeed users have a speed
> feeling. Please consider that on eavilly loaded servers, with 40 and more
> users, some are running gcc, others g77, others g++ compilations, someone
> runs pine or mutt or kmail, and netscape, and mozilla, and emacs (someone
> form xterm kde or gnome), and and
> and... You can have also 4/8 CPU butthey are not infinite ;) (but I talk
> mainly thinking of dualAthlon systems).
> there is a lot of memory and disk I/O.
> This is not a strange scenary on the interactive servers used at SNS.
> Here preempt has a too high price

That's why preempt is a compile time option, btw.

> > By the way, have you measured the cost of -preempt in practice?
>
> Yes, I did a lot of tests, and with current preempt patch definitelly
> I was seeing a too big performance loss.

Have you tried with stock 2.4.17 or with additional patches?
2.4.17-rc2aa2 (10_vm-21)?

The later make big differences in throughput for me (with and without
preempt).

I am under preparation of some numbers.
Anybody want some special tests?
dbench (yes, I know...) with and without MP3 during run
latencytest0.42-png
bonnie++
getc_putc

Thank you for all your serious answers. This was definitely not intended as a
flamewar start.

-Dieter
--
Dieter Nützel
Graduate Student, Computer Science

University of Hamburg
Department of Computer Science
@home: Dieter.Nuetzel@hamburg.de