Re: [Xen-devel] Scheduling of I/O domains

["G. Milos" <gm281@xxxxxxxxxxxxxxxx>]

> [sorry for the delayed reply]
>
> as keir pointed out the problem is in the wakeup function and in
> particular with a BVT hack.
>
> BVT has the notion of a context switch allowance, i.e., the minimum time
> a task is allowed run before it gets preempted, to avoid context switch
> thrashing (ctx_allow=5ms in sched_bvt.c). after this time a new run
> through the scheduler is performed
>
> in our BVT implementation we extend this slightly in that if there is
> only one runable task we expand the context switch allowance to 10 times
> the normal amount in order to avoid to many runs through the scheduler.
>
> The old (i.e., 1.2) BVT implementation would check on waking up another
> domain if the current task already used up the ctx_allow and if it had
> would force an immediate run through the scheduler (therefor ignoring
> the the expanded context switch allowance).
>
> In the -unstable implementation this is not the case anymore as the BVT
> scheduling function reports back a time value for the next run through
> the scheduler
True.

> and no hook is provided into specific scheduler
> implementations when a domains is unblocking. Therefor, if your CPU hog
This is not the case.
Firstly the expanded ctx_allow does not affect the minimum time that a 
domain is scheduled to run as it is allways set to 5ms (line 496 in 
sched_bvt.c).
Secondly, when a domain is unblocking the scheduler specific wakeup 
function decides if to force immediate run through the scheduler (line 
232 sched_bvt.c)
> is the only runable task when it is scheduled it will run 10*ctx_allow
> (50ms) irrespective of other tasks becoming runnable during that time
> (ie. your IO tasks). in the worst case the IO tasks then have to wait
> for 50ms rather than 5ms before they get scheduled.
It will still be 5ms only. I have some scheduling traces which
confirm that. Still we want something close to 0 not 5ms.

> as a quick fix could you comment out line 366-371  (which extends the
> context switch allowance if there is only one task running) in
> sched_bvt.c and  try your experiment again.
Also worth noting is that it is idle domain for which the time quantum
is expanded. If there are more runnable tasks (ie at least one "normal") 
then ctx_allow will be 5ms (line 472 sched_bvt.c)
> The proper fix should be a call into the scheduler if a task unblocks,
> which shouldn't be too hard to add.
That's right. I will try to do that now.

Cheers
Gregor


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