[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] credit2 question

On 24/01/13 10:42, Jan Beulich wrote:
On 24.01.13 at 11:07, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
On 24/01/13 10:06, Jan Beulich wrote:
On 24.01.13 at 10:49, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
On 24/01/13 07:40, Jan Beulich wrote:

I'm getting puzzled by the second c2t() invocation in
csched_runtime(): Why is the difference of credits being passed
here? Doesn't that (unless svc->credit is non-positive, i.e. in all
but unusual cases) guarantee time > ntime, and particularly
allow for negative ntime?
Ah, right -- yes, if the other guys' credit is positive, "ntime" is
guaranteed to be lower.  Since c2t() involves integer division, it would
definiteyl be good to get rid of the extra call if we can.

My general principle is to make the code clear and easily readable
first, and then do optimization afterwards -- in this case I just never
came back and did the optimization step.
Oh, I wasn't thinking of just the optimization. It seemed wrong to
me to do the subtraction there in the first place: "time" is being
calculated from a plain value, so why would "ntime" be calculated
from a delta?
Ah, right -- so the idea here was to run until snext->credit was equal
to svc->credit.  That's why the delta.
Which then means that under normal circumstances you would
always only run each vCPU for CSCHED_MIN_TIMER, which
seems quite odd. Wouldn't it be more fair to do e.g.

             if ( time > ntime )
                 time = (time + ntime) / 2;

since otherwise at the expiry of the time the two vCPU-s have
equal credit, whereas you would generally expect a vCPU that
just finished running to have lower credit than the next one to

But as you validly said earlier, avoiding the c2t() in cases where we
can tell up front that "time" would end up below CSCHED_MIN_TIMER
(particularly zero or negative) would be desirable. I'd prefer to
leave doing that to you though.

Hmm, actually doing the stuff with MIN and MAX timer is more tricky; I forgot that vcpus with different weights burn credit at different rates, so we'd have to pre-calculate t2c(MIN) and t2c(MAX) for each vcpu. That sounds like a bit more code than I'm up for ATM (and more complexity than I'd like to add unless there's a measurable benefit, which I don't have time to measure ATM); but I can certainly get rid of the second c2t().


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.