Re: [uml-devel] Q: UML thread communication - scheduling oddness

[Joe Marzot <gmarzot@nortelnetworks.com>]

That's an excellent theory esp. ince we do not see the problem typically 
on a host with more memory. I presume there are tricks we can play to 
get our memory not to be swapped out (recall a discussion on that here).

Could you add a few sentences though on the delivery mechnisms for the 
the packets to the kernel thread and to userspace. I saw a poll() on the 
pollfds array which has the taps fd in it but this is a non-blocking 
poll and I could not trace to how often it is called and what triggers 
it - I also saw the sigio handler but I could not see who was generating 
the SIGIO - is that how the userspace thread is woken up?

If there is some thing else I can read to get some background that would 
help...otherwise its back to the source of course.

thank you kindly, Giovanni

Jeff Dike wrote:
> gmarzot@nortelnetworks.com said:
>  > That is, if I run a little busy loop
>  >   /mnt/plankton/stress --cpu 1 --io 1
>  > on the server side, the round trip time for messages is greatly
>  > *improved*. without this activity and a basically dormant server side
>  > UML the round trip times show considerable variabilty with gaps on the
>  >  order of seconds.
> 
> How's this for a theory:  The server's host is short on memory.  A UML 
> which
> is answering pings every second or so can be considered pretty idle, and 
> thus
> a good candidate to have its pages swapped out.  So, this happens, and when
> a ping comes in, UML needs to be swapped back in, causing your long and
> unpredictable delays.
> 
> Running a busy loop in the UML makes it busy from the point of view of the
> host and prevents it from being swapped out, giving you nice ping 
> latencies.
> 
>                                 Jeff
> 
> 





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