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 23:18, Henrik Nordstrom wrote:
> > That's my only argument as to why it should be included:  mlocked
> > kernels behave more predictably in a hosting environment!
>
> In principle I agree, but it is somewhat of a corner case. For most
> practical uses you should be able to get the same results with a correctly
> tuned VM on the host and having sufficient amount of memory to allow for a
> decent buffer/cache in addition to the UML memory usage. Only if you
> overbook the memory usage (including buffer/cache requirements on
> the host) should there be significant problems.

I'm guessing the people who are going to be most concerned with UML's 
performance are ISPs, who are trying to get the best performance from their 
expensive U of data centre rack space; maybe this is a niche case at the 
moment.  

I can believe that the kernel is making compromises in terms of buffer 
requirements after the 50-60% full mark (which is about when we've seen 
swapping as you predicted).  However after buying a certain amount of RAM for 
your box, it is *much* cheaper to compensate for the higher disc demands with 
arrays of fast discs than it is to put more RAM in.  This doesn't make the 
kernel perform well from that point of view, but it means customers get the 
performance and price they expect :)

cheers,

- -- 
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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