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

Andrea Arcangeli
2002-01-14T11:33:19Z

On Sun, Jan 13, 2002 at 01:11:21PM -0500, Robert Love wrote:
> On Sun, 2002-01-13 at 10:18, jogi@planetzork.ping.de wrote:
>
> > No, I use a script which is run in single user mode after a reboot. So
> > there are only a few processes running when I start the script (see
> > attachment) and the jobs should start from the same environment.
> >
> > > What happens when you do the same test, compiling one kernel under multiple
> > > different kernels?
> >
> > That is exactly what I am doing. I even try to my best to have the exact
> > same starting environment ...
>
> So there you go, his testing is accurate. Now we have results that
> preempt works and is best and it is still refuted. Everyone is running
> around with these "ll is best" or "preempt sucks throughput" and that is

assuming the report can be trusted this is not the test where we can
measure a throughput regression, this is a VM intensive test and nothing
else. Swap load.

In short, run top and check you've 100% system load and cpus are never
idle or in userspace, and _then_ it will most certainly get an interesting
benchmark for -preempt throughput.

Furthmore the whole comparison is flawed, just -O(1) is as broken as
mainline w.r.t. the scheduling point, and -aa has the right scheduling
point but not the -O(1) scheduler, so there's no way to compare those
numbers at all. If you want to make any real comparison you should apply
-preempt on top of -aa.

Assuming it is really -preempt that makes the numbers more repetable
(not the fact -O(1) alone has the broken rescheduling points), this
still doesn't proof anything yet, the lower numbers are most certainly
because those tasks getting the page faults get rescheduled faster, -aa
didn't do more cpu work, it just had the cpus more idle than -preempt
apparently, this may be the indication of an important scheduling point
missing somewhere, if somebody could run a lowlatency measurement during
a swap intensive load and send me the offending IP that could probably
be addressed with a one liner.

Andrea