[uml-devel] Re: [uml-user] Network lags

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

David Cannings wrote:
>>>Anyone else who's running lots of UMLs with interactive sessions may have
>>>noticed the following symptoms which this patch cures:
>>>
>>>  * irregular ping times to UMLs-- huge delay for the first ping, a few
>>>      normal, a couple of long delays etc.;
> 
> The huge delay is often related to ARP, a fundamental part of 802.3/true
> ethernet.

Indeed, but when logging back on to an ssh session etc. it's very 
irritating for customers to find their machine takes seconds to "wake 
up" while the 2.4 kernel swaps stuff back in.

[snip]
> The problem with "just activating that option" is that many people will do
> it blindly, unaware of the serious implications it could have to their host.
> Whilst mlocking an area of memory may not affect you now, or in the next ten
> minutes, it could cause trouble in three weeks when you start doing
> something on the host that is memory intensive.  In the last three years of
> running a remote server I have only had to turn up on site to fix it twice.
> Both were very silly things, one that I'd "just activated" to see what it
> would do.  It cost me a rather expensive journey half way across the country
> to fix.

Since writing that post back in October, we've deployed the mlock patch 
across two rackfuls of hosts, and we've never had a host kernel "just 
crash" (well we have, but that was down to something else <g>): longest 
uptime on one group of our hosts is 100 days.  Yes it was experimental 
but we did a bit of testing first.

 From this experience; I would disagree that using lots of mlocked 
memory inherently destabilises the machine.  I'm sure it *can* but this 
goes for a lot of things.

[snip]
> The idea behind UML is to create a separate environment from the host,
> constructing a UML kernel that can interfere so badly with the host is the
> total opposite.

It's very easy for a UML kernel to trash the host, and I'm not sure 
anyone knows what "the idea behind UML" is :-)

I can see Jeff's point about the mlock patch despite the fact that it 
solved a major problem for us with deploying UML in a hosting 
environment.  Jeff's proposed new memory management features will give 
us more flexibility than my simple on/off mlock patch, and allows him to 
wash his hands of these grubby management decisions.

cheerio,

-- 
Matthew Bloch                             Bytemark Hosting
                         http://www.bytemark-hosting.co.uk/
                 phone UK: 0845 004 3 004 US: 1-877 BYTEMAR
           Dedicated Linux hosts from 15ukp ($26) per month



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