Re: Two naive questions and a suggestion

[Vladimir Dergachev <vladimid@blue.seas.upenn.edu>]

On 23 Nov 1998 jfm2@club-internet.fr wrote:

> 
> > 
> > Hi,
> > 
> > On 19 Nov 1998 00:20:37 -0000, jfm2@club-internet.fr said:
> > 
> > > 1) Is there any text describing memory management in 2.1?  (Forgive me
> > >    if I missed an obvious URL)
> > 
> > The source code. :)
> > 
> 
> I knew about it.  :)  And this is not an URL :)
> 
> > > 2) Are there plans for implementing the swapping of whole processes a
> > >    la BSD?
> > 
> > Not exactly, but there are substantial plans for other related changes.
> > In particular, most of the benefits of BSD-style swapping can be
> > achieved through swapping of page tables, dynamic RSS limits and
> > streaming swapout, all of which are on the slate for 2.3.
> > 
> 
> The problem is: will you be able to manage the following situation?
> 
> Two processes running in an 8 Meg box.  Both will page fault every ms
> if you give them 4 Megs (they are scanning large arrays so no
> locality), a page fault will take 20 ms to handle.  That means only 5%
> of the CPU time is used, remainder is spent waiting for page being
> brought from disk or pushing a page of the other process out of
> memory.  And both of these processes would run like hell (no page
> fault) given 6 Megs of memory.
> 
> Only solution I see is stop one of them (short of adding memory :) and
> let the other one make some progress.  That is swapping.  Of course
> swapping can be undesiarable in work stations and that is the reason I
> suggested user control about MM policy be it by recompiling, by /proc
> or by module insertion.
> 
> In 96 I asked for that same feature, gave the same example (same
> numbers :-) and Alan Cox agreed but told Linux was not used under
> heavy loads. That means we are in a catch 22 situation: Linux not used
> for heavy loads because it does not handle them well and the necessary
> feaatures not implemented because it is not used in such situations.
> 
> 
> And now we are at it: in 2.0 I found a deamon can be killed by the
> system if it runs out of VM.  Problem is: it was a normal user process
> who had allocatedc most of it and in addition that daemon could be
> important enough it is better to kill anything else, so it would be
> useful to give some privilege to root processes here.
> 
> I think this ends my Christmas wish list.  :)

what about this solution: write a small program the monitors for programs
(via /proc) that swap a lot. If this happens make them get "wide
slice" time - i.e. send SIGSTOP to everybody and SIGRESUME/SIGSTOP with
interval 0.5-1 sec. This would have the concurency you want and will also
eliminate the swapping (mostly). Since you need this heavy load anyway
I don't think your programs will complain about long delays - for all they
know this time is taken up swapping.

And this being a userspace program will allow you to fine tune this as
much as you want, all the way to OOM/OOT killer that will pop-up a nice
box on the terminal and ask what you want to do. 


                     Vladimir Dergachev


> 
> -- 
> 			Jean Francois Martinez
> 
> Project Independence: Linux for the Masses
> http://www.independence.seul.org
> 
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