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SubjectRe: VM
and the problem with trying to do the perfect thing in every situation is
that you need to predict all the situations so that you have the right
response for each one.

it gets even worse when you have a mix of loads (parts of several
different basic situations).
this is why a simpler solution that may not be quite as good in several of
the simple situations can end up being a winner in the real world (where
you almost always have a mix of situations)

also if you have a lot of special cases you need to choose between you
can easily end up spending more time selecting the proper mode then you
save by being in that mode

one thing that the entire 2.[34] VM hassle has shown is that the VM
hackers don't have a good handle on the real world loads along with a way
to reasonably duplicate those loads in testing (if anyone had any idea
what sort of VM problems 2.4 would have they would have been resolved in
the 2.3 series, right?) This makes simple/general solutions better then
attempts to define and select a bunch of special purpose solutions.

from the looks of things Rik's VM is getting to the point where it is
almost covering enough special cases to be practical, but for the reasons
I gave above I have less confidence in it then in the AA VM for the near
future. Long term may be a different story however.

One thing that I think is needed for future 2.4/2.5 before much more
VM development can be done is some significant instramentation of the
existing VM to gather data on real-world loads along with some simulater
to be able to recreate them. without this sort of data and tools the best
that the VM hackers can do is to test it on the machines and loads they
see and hope that they cover enough cases to make the special casing
approach work, otherwise we keep running the risk of the same type of
problems with the next implementation.

in part this is a chicken-and-egg problem, until the new VM gets extensive
real-world testing it won't run into all the corner cases to be able to
see if it works well but this also means that when released on the general
user base it will break peoples servers. thus the need for better
documentation and instramentation of the real world loads (becouse they
are frequently NOT the 'best' way of doing things and in some cases they
are downright bad ways)

David Lang



On Tue, 16 Oct 2001, safemode wrote:

> Date: Tue, 16 Oct 2001 01:08:02 -0400
> From: safemode <safemode@speakeasy.net>
> To: linux-kernel@vger.kernel.org
> Subject: Re: VM
>
> I think it was said earlier that we're dealing with definitions of stability
> and performance.
> If you look at stability as being able to depend on an effect given a cause,
> then andrea's has been said to be more stable in that sense. If you look at
> performance being the amount of different situations that the vm is stable
> and everything else being a lower priority, then andrea's vm is better
> performing.
>
> Rik's vm is performance first, meaning it tries to do what's best for each
> situation and basically the difference is that rik's vm has more variables
> effecting what happens. This treats everything differently but it means that
> each situation is dealt with personally instead of trying to blanket each
> like situation. That basically destroys our previous definition of
> stability. So we make a new one. Stability here deals with how much the
> system needs to stop other things for VM things. Of course the not
> corrupting things and crashing things are implied to both definitions.
> For instance though, when you swap, that takes time away to write to disk.
> This can take longer than a complex way of re-arranging pages and removing
> pages in ram.
>
> It seems like andrea's vm is more tuned for systems that do the same things
> over and over, like a server. And rik's vm is more tuned for systems that
> you dont know what is going to be run or is running numerous programs that
> have no real regularity.
>
> And complex does not mean smart, but it doesn't mean it can't be smart. When
> you're dealing with something as complex as a VM, using a simplistic approach
> may just be too limiting in the end and I think many people are seeing that
> when they say programs are more responsive in alan's kernel and memory usage
> is more efficient. It doesn't seem very logical to make the VM do B when you
> do A on a multiprocessing system unless the environment is exactly the same
> every time you do A because what's good at one time doesn't mean it's good
> the second or third time you do it either due to memory limitations or other
> applications requiring different things of the VM.
>
> I'm kind of picturing the two VM's like the two parts of our brain. The
> brain stem (sometimes called the reptillian brain) and the cerebrum. Your
> reptillian brain is quite fast at reacting and can make a few decisions on
> what to do based on a few specific variables. The cerebrum is slower at
> those same tasks but it better manages those tasks, based on many more
> variables, so that the reaction is not too much or too little so that the
> next thing that happens is in a better position than what the reptillian
> brain would have left for it.
>
> Of course being able to do more means you have opened yourself up to more
> problems. I wont speak for all ac branch users, but i feel that the more
> complex way of handing memory is a better choice because it's a function of
> the kernel that demands a complex solution. A simplistic solution is too
> limited, it would be like reacting from your brain stem and overreacting
> instead of using your higher logic and taking a more educated reaction.
>
> And that's all the contraversy, deciding if 2.4's VM demands a complex
> solution that handles each situation uniquely, or it can have a simple
> solution that handles a wide range fairly good.
>
> Perhaps aiming for a simplistic VM should be the goal of 2.5 from the
> beginning ( as if it wasn't), that way you can build everything else around
> it and avoid all this vm trouble that 2.4 has been plagued with since the mid
> 2.3 days.
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