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Date: Thu, 25 Oct 2001 11:10:25 -0200 (BRST)
From: Rik van Riel <riel@conectiva.com.br>
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To: Steven Butler <stevenb1@bigpond.net.au>
Cc: <linux-kernel@vger.kernel.org>
Subject: Re: Memory Paging and fork copy-on-write semantics
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On Thu, 25 Oct 2001, Steven Butler wrote:

> I have been making use of copy-on-write semantics of linux fork to
> duplicate a process around 100+ times to generate client load against a
> server.  The copy-on-write allows me to run many more processes without
> swap thrashing than I'd otherwise be able to.  The client code is in
> perl, so the process sizes are in the MBs.  Using this technique I only
> need about 2 MB per user, with around 5.5 MB shared.

	[snip COW undone on swapout, leading to thrashing]

> Is this expected and reasonable behaviour?

Absolutely not, this is not supposed to happen.

>  Is it possible for pages to remain shared, even when they are swapped
> to disk?

I think this already happens in the -ac kernels, probably
in -linus too (though I'm not 100% sure).

> Does that already happen anyway, meaning my analysis of the situation
> is off base?

Possible, but it's also possible you ran into a real bug,
it would be interesting to debug this further...

I wouldn't rule out a bug with the remove-from-swapcache
logic on swapin, either.

regards,

Rik
-- 
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