On Fri, 8 Jun 2001, Tobias Ringstrom wrote:

On Fri, 8 Jun 2001, Mike Galbraith wrote:

quoted

On Fri, 8 Jun 2001, Tobias Ringstrom wrote:

quoted

On Fri, 8 Jun 2001, Mike Galbraith wrote:

quoted

I gave this a shot at my favorite vm beater test (make -j30 bzImage)
while testing some other stuff today.

Could you please explain what is good about this test?  I understand that
it will stress the VM, but will it do so in a realistic and relevant way?

Can you explain what is bad about this test? ;)  It spins the same VM wheels

I think a load of ~30 is quit uncommon, and therefor it is unclear to me
that it would be a test that would be repesentative of most normal loads.

It's not supposed to be repesentative.  It's supposed to take the box
rapidly (but not instantly) from idle through lo->medium->high and
maintain solid throughput.

quoted

as any other load does.  What's the difference if I have a bunch of httpd
allocating or a bunch of cc1/as/ld?  This load has a modest cachable data
set and is compute bound.. and above all gives very repeatable results.

Not a big difference.  The difference I was thinking abount is the
difference between spawning lots of processes allocating, using and
freeing lots of memory, compared to a case where you have a few processes
touching a lot of already allocated pages in some pattern.  I was
wondering whether optimizing for your case would be good or bad for the
other case.  I know, I know, I should do more testing myself.  And I
should probably not ask you, since you really really like your test,
and you will probably just say yes... ;-)

It's not a matter of optimizing for my case.. that would be horrible.
It's a matter of is the vm capable of rapid and correct responses.

At home, I'm running a couple of computers.  One of them is a slow
computer running Linux, serving mail, NFS, SMB, etc.  I'm usually logged
in on a couple of virtual consoles.  On this machine, I do not mind if all
shells, daemons and other idle processes are beeing swapped out in favor
of disk cache for the NFS and SMB serving.  In fact, that is a very good
thing, and I want it that way.

Another maching is my desktop machine.  When using this maching, I really
hate when my emacsen, browsers, xterms, etc are swapped out just to give
me some stupid disk cache for my xmms or compilations.  I do not care if a
kernel compile is a little slower as long as my applications are snappy.

How could Linux predict this?  It is a matter of taste, IMHO.

I have no idea.  It would be _wonderful_ if it could detect interactive
tasks and give them preferencial treatment.

quoted

I use it to watch reaction to surge.  I watch for the vm to build to a
solid maximum throughput without thrashing.  That's the portion of VM
that I'm interested in, so that's what I test.  Besides :) I simply don't
have the hardware to try to simulate hairy chested server loads.  There
are lots of folks with hairy chested boxes.. they should test that stuff.

Agreed.  More testing is needed.  Now if we would have those knobs and
wheels to turn, we could perhaps also tune our systems to behave as we
like them, and submit that as well.  Right now you need to be a kernel
hacker, and see through all the magic with shm, mmap, a bunch of caches,
page lists, etc.  I'd give a lot for a nice picture (or state diagram)
showing the lifetime of a page, but I have not found such a picture
anywhere.  Besides, the VM seems to change every new release anyway.

quoted

I've been repeating ~this test since 2.0 times, and have noticed a 1:1
relationship.  When I notice that my box is ~happy doing this load test,
I also notice very few VM gripes hitting the list.

Ok, but as you say, we need more tests.

quoted

quoted

Isn't the interesting case when you have a number of processes using lots
of memory, but only a part of all that memory is beeing actively used, and
that memory fits in RAM.  In that case, the VM should make sure that the
not used memory is swapped out.  In RAM you should have the used memory,
but also disk cache if there is any RAM left.  Does the current VM handle
this case fine yet?  IMHO, this is the case most people care about.  It is
definately the case I care about, at least. :-)

The interesting case is _every_ case.  Try seeing my particular test as
a simulation of a small classroom box with 30 students compiling their
assignments and it'll suddenly become quite realistic.  You'll notice
by the numbers I post that I was very careful to not overload the box in
a rediculous manner when selecting the total size of the job.. it's just
a heavily loaded box.  This test does not overload my IO resources, so
it tests the VM's ability to choose and move the right stuff at the right
time to get the job done with a minimum of additional overhead.

I did not understand those numbers when I saw them the first time.  Now, I
must say that your test does not look as silly as it did before.

[snip.. save a tree]

Why isn't user+system+idle == real?  SMP?

Good question, no smp (sniff) here.

quoted

Tunables aren't really practical in VM (imho).  If there were a dozen
knobs, you'd have to turn a dozen knobs a dozen times a day.  VM has
to be self regulating.

Yes, that is of course the goal, but I'm suggesting that we would reach
the goal of a self-optimizing VM faster, if there were tunables to play
with.  The human brain is a very good optimizer.

You bet!  The CPU is a stupid robot.  I've tried to think up good generic
tunables, and failed.  This is something that more folks should give some
thought.  Maybe someone will think of knobs that _are_ practical.

quoted

In case you can't tell (the length of this reply) I like my fovorite
little generic throughput test a LOT :-)

Point taken.  :-)

	Cheers,

	-Mike

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