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SubjectRe: Subtle MM bug
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Date
Rik van Riel <riel@conectiva.com.br> writes:

> On 7 Jan 2001, Zlatko Calusic wrote:
>
> > Things go berzerk if you have one big process whose working set
> > is around your physical memory size.
>
> "go berzerk" in what way? Does the system cause lots of extra
> swap IO and does it make the system thrash where 2.2 didn't
> even touch the disk ?
>

Well, I think yes. I'll do some testing on the 2.2 before I can tell
you for sure, but definitely the system is behaving badly where I
think it should not.

> > Final effect is that physical memory gets extremely flooded with
> > the swap cache pages and at the same time the system absorbs
> > ridiculous amount of the swap space.
>
> This is mostly because Linux 2.4 keeps dirty pages in the
> swap cache. Under Linux 2.2 a page would be deleted from the
> swap cache when a program writes to it, but in Linux 2.4 it
> can stay in the swap cache.
>

OK, I can buy that.

> Oh, and don't forget that pages in the swap cache can also
> be resident in the process, so it's not like the swap cache
> is "eating into" the process' RSS ;)
>

So far so good... A little bit weird but not alarming per se.

> > For instance on my 192MB configuration, firing up the hogmem
> > program which allocates let's say 170MB of memory and dirties it
> > leads to 215MB of swap used.
>
> So that's 170MB of swap space for hogmem and 45MB for
> the other things in the system (daemons, X, ...).
>

Yes, that's it. So it looks like all of my processes are on the
swap. That can't be good. I mean, even Solaris (known to eat swap
space like there's no tomorrow :)) would probably be more polite.

> Sounds pretty ok, except maybe for the fact that now
> Linux allocates (not uses!) a lot more swap space then
> before and some people may need to add some swap space
> to their system ...
>

Yes, I would say really a lot more. Big diffeence.

Also, I don't see a diference between allocated and used swap space on
the Linux. Could you elaborate on that?

>
> Now if 2.4 has worse _performance_ than 2.2 due to one
> reason or another, that I'd like to hear about ;)
>

I'll get back to you later with more data. Time to boot 2.2. :)
--
Zlatko
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