Re: Scheduling of I/O domains

[Rob Gardner <rob.gardner@hp.com>]

G. Milos wrote:
> 
> To target the unfairness I am developing a modification of BVT (I called 
> it Fair Borrowed Virtual Time [FBVT]). You can enable it by supplying 
> "sched=fbvt" command to Xen at the startup. The scheduler is under 
> development and it needs some tweaking to get the best performance (that 
> is what I am working on at the moment). It would be very helpful if you 
> could email me with the results of your tests for FBVT.


I tried booting xen with "sched=fbvt" in the command line in grub.conf. 
It didn't change the results at all. And why would it? We are not 
dealing with an "I/O bound domain" here, but rather with an "I/O 
domain", two very different things.

It seems to me that this problem doesn't have anything to do with the 
choice of scheduling policy or parameters; It is about when the 
scheduler is called. It appears as though the xen cpu scheduler 
currently only runs when the hardware timer ticks. It does not run when 
an external interrupt happens. So there is a large latency introduced to 
I/O interrupts, and this limits I/O performance. Changing the scheduler 
algorithm won't help this.

The only way to avoid this is to immediately dispatch the I/O domain 
responsible for a given I/O interrupt as soon as that interrupt occurs. 
This means giving I/O domains with pending interrupts scheduling 
priority over any "regular" domains. Just as in a "normal" operating 
system, interrupt service routines must complete before any user 
processes are executed. Otherwise, latencies are introduced that kill 
I/O performance.

Rob Gardner



-------------------------------------------------------
This SF.Net email is sponsored by BEA Weblogic Workshop
FREE Java Enterprise J2EE developer tools!
Get your free copy of BEA WebLogic Workshop 8.1 today.
http://ads.osdn.com/?ad_id=4721&alloc_id=10040&op=click