-----Original Message-----
From: Christoph Rohland [mailto:[EMAIL PROTECTED]]
Sent: Friday, November 03, 2000 7:54 AM
To: Rik van Riel
Cc: Jonathan George; '[EMAIL PROTECTED]';
'[EMAIL PROTECTED]'
Subject: Re: 2.4.0-test10 Sluggish After Load


Hi Rik,

>On Wed, 1 Nov 2000, Rik van Riel wrote:
>> The 2.4 VM is basically pretty good when you're not
>> thrashing and has efficient management of the VM until
>> your working set reaches the size of physical memory.
>> 
>> But once you hit the thrashing point, the VM falls
>> flat on its face. This is a nasty surprise to many
>> people and I am working on (trivial) thrashing control,
>> but it's not there yet (and not all that important).
>
>I looked into this argument a little bit further: 
>In my usual stress tests 12 processes select a random memory object
>out of 15 to mmap() or shmat() it and then access it serially. Each
>segment is 666000000 bytes and I have 8GB of memory. So at one time
>there are at most 666000000*12 bytes = 7.45GB memory attached and in
>use. So I do not see that the machine qualifies as thrashing. Of
>course the memory pressure is very high all the time since we have to
>swap out unused segments.
>
>But the current VM does not behave good at all on that load.
>
>Greetings
>               Christoph

I wonder how much of that memory is actually being used by your processes.
My guess is that it's not the whole thing (unless you are running on a 64bit
architecture).

--Jonathan--

P.S. ------------------ THRASH REDUCTION THOUGHTS -----------------

I have given some thought to the issue of thrash recovery, and I have come
up with a simple (conceptually) way to implement a fair thrash reduction
algorithm.  However, since I know a minimal amount about the low level
implementation of the Linux scheduler and VM beyond behavioral
characteristics I would love to have some feedback since it should at least
clear up some of my questions about kernel internals.

My idea:
        If THIS process is paged in for consecutive scheduler slices
        Then
                Allow THIS process to run for XX% of the time
                        it took to swap THIS process in.

Where:
        THIS -- is the current runnable process.
                (multiple processes for SMP)
        XX% -- is the thrash recovery bias.
                (from 10-150% is probably reasonable)
        time it took to swap --
                -- combined system swap time from fault.
                        (until execution for this scheduler slice)

What do you think?
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