Re: [uml-devel] [PATCH] Locking user-mode kernel RAM into host physical RAM

[Matthew Bloch <matthew@bytemark.co.uk>]

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On Monday 08 December 2003 19:01, Jeff Dike wrote:
> As far as I'm concerned, this is papering over the symptoms rather than
> fixing a real problem.
>
> If you're really not overselling memory, then the host should not be
> swapping. If it is, then that's the problem.

Hi Jeff, I don't think this is true.  The 2.4 kernel does appear to swap out 
applications when (e.g.) a large file copy is underway, and the 2.6 kernel 
has been tweaked to get around this problem:

http://kerneltrap.org/node/view/1044

We've not had enough time to try 2.6 on a production server as a host kernel, 
but I'd be interested if you did know a way of tweaking a 2.4 host kernel to 
guarantee RAM-locked pages to user-mode kernels; Matt Ayres mentioned "some 
interactivity and vm patches", could you elaborate on that please Matt?  For 
various reasons, we use a Redhat 2.4.20 kernel which I believe has had its 
memory management fiddled with.

I like the sound of the more dynamic memory arrangement and manager daemon for 
UMLs, sounds like a good way to squeeze more customers onto a host machine.  
But if for the moment the only thing it should do is to keep the host from 
swapping, I know from a few our customers that mlocking has given more 
predictable behaviour from guest kernels, especially if the host load spikes 
for a few minutes.  That's my only argument as to why it should be included: 
mlocked kernels behave more predictably in a hosting environment!  But 
obviously I'm biased; if I can get the same effect without the patch on a 2.4 
host kernel I'd agree it's moot.

- -- 
Matthew Bloch                             Bytemark Hosting
                                  tel. +44 (0) 8707 455026
                        http://www.bytemark-hosting.co.uk/
          Dedicated Linux hosts from 15ukp ($26) per month
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