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Scheduler "Burst" (was Re: [Xen-devel] domU config file options for sch

To: Thomas Goirand <thomas@xxxxxxxxxx>
Subject: Scheduler "Burst" (was Re: [Xen-devel] domU config file options for scheduling)
From: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Date: Thu, 10 Dec 2009 18:28:26 +0000
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[Starting a separate thread for a separate topic]

Regarding the "burst" feature, as I said at the Summit, I think as
described it's too specific for your use case.  I'm sure it would be
useful for you, but how many hosting providers would want to use that
exact interface?  Maybe they'd like a similar feature but with slight
changes; maybe they'd like to solve the same problem in a completely
different way.

If your "burst" time is on the order of seconds, it seems to me that a
demon running in dom0, monitoring VM cpu usage every second or so, and
adjusting the cap / weight could have the same overall effect using
the existing interfaces.  Having these kinds of policies in a demon is
a lot more flexible, as the demon can be stopped / modified /
rewritten to the particular policy someone wants without having to
change Xen at all.


On Thu, Dec 10, 2009 at 7:58 AM, Thomas Goirand <thomas@xxxxxxxxxx> wrote:
> Also, I had a chat with George about having a kind of "burst" feature
> added to the credit scheduler. My idea is that, by default, someone
> would setup a credit-burst-time, and a credit-burst-cap, to any VM that
> has a a credit-cap set. That could be set this way in the domU config file:
> shedcreditweight = 20
> shedcreditcap = 50
> shedbursttime = 5000 <--- 5 seconds max burst CPU time
> shedburstcap = 80 <--- 80% CPU max during the burst
> shedrecovertime = 2000 <--- 2 second recovery time
> Whenever a VM goes over its shedcreditcap, the burst-cap would be used
> for a time no longer than the credit-burst-time. When this time is over,
> then shedcreditcap would be used, until the VM uses less than
> shedcreditcap for a time longer than shedrecovertime.
> I do believe that this kind of scheduling would be REALLY useful for
> avoiding that a VM abuses the CPU usage. For people doing Xen VM hosting
> business, or cloud computing, that would be just great. The values that
> I wrote above would be a typical use.
> I had a look myself in the scheduler code, and it's quite not obvious
> where to patch. I think I have understood the burncredit() system, but I
> didn't get where the values are read for the cap and all. Did I
> understood well that Xen uses 30ms cycles for the credit scheduler? If
> that is the case, then I believe that computing what cap values to use
> depending on the burst parameters for a given 30ms scheduling cycle
> wouldn't be such a big overhead.
> If nobody wants to implement this, can someone gives me some pointers in
> the sched_credit.c and all the .h structures? If I do stupid trials and
> errors in my implementation at first, because I'd be a beginner at
> patching Xen, would you guys have enough time to explain, and excuse a
> newbie? Would that kind of patch be accepted if proven to work?
> Cheers,
> Thomas Goirand
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