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

[Joe Marzot <gmarzot@nortelnetworks.com>]

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.

thinking about this a bit more...some doubts...note: the regular 'pings' 
which are serviced purely by kernel resident code/ kernel thread do not 
exhibit the delays. It is only when the communication involved a 
userspace app as the responder to the 'app-level-ping' that the delays 
come. Is the memory for the userspace app seperately swappable from that 
of the kernel thread? If so then the theory still makes sense, since the 
kernel thread likely has more going on so that is less likely to swap. 
If not, I am still confused.

-GSM

> 
> 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
> 
> 





-------------------------------------------------------
This SF.Net email is sponsored by OSTG. Have you noticed the changes on
Linux.com, ITManagersJournal and NewsForge in the past few weeks? Now,
one more big change to announce. We are now OSTG- Open Source Technology
Group. Come see the changes on the new OSTG site. www.ostg.com
_______________________________________________
User-mode-linux-devel mailing list
User-mode-linux-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/user-mode-linux-devel