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

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Subject: Re: Scheduler "Burst" (was Re: [Xen-devel] domU config file options for scheduling)
From: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Date: Fri, 11 Dec 2009 10:47:01 +0000
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Thomas Goirand <thomas@xxxxxxxxxx>
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As I understand it, Thomas wants a new "boost cap" that behaves
differently than the current cap. The current cap gives x% of cpu,
period. What he wants (as I understand it) is a time-based cap, that
will allow a VM to run for y% (where y>x) for some amount of time, and
thereafter limit it to x% until some other conditions are met.

IIRC, the problem he's trying to solve is this. He's a hosting provider,
and hosted VMs sometimes have a legitimate reason to use (say) 80% of
the cpu for short periods of time. He wants to allow a VM to do that.
However, he doesn't want "misbehaving" VMs to use 80% of the cpu for
long periods of time. So if a VM has used a high amount of CPU for a
sustained period (5 seconds in his example), he wants it to be capped
down to something like 50%.

Anyway, Thomas: before trying to work with the scheduler (which is an
exercise in frustration even for experienced OS coders), I think writing
a daemon to wake up every 1000ms (perhaps every 500ms), read the cpu
usage stats (you could look at xentop to see how this is done), and set
the weight and/or cap would be a better way to go. If you wrote it then
it would also be a lot easier for you to refine the policy if it didn't
fit your needs.

I'd start by looking at how xentop reads the domain info. Feel free to
ask questions on xen-devel if you run into trouble with the interface.


Tian, Kevin wrote:
> I don't quite understand the problem here. Does that mean current 'cap'
> implemented in credit scheduler is not a real cap, and some VM may
> still go over specified cap to grab cpu for unexpected time, and then
> Thomas wants to add another 'burst-cap' to limit that abused behavior?
> If that the case, we should fix current implementation.
> Simply back to requirement, I agree with George that second-based
> restriction can be easily done in daemon.
> Thanks,
> Kevin
>> From: George Dunlap
>> Sent: 2009年12月11日 2:28
>> [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.
>> -George
>> 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
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>> http://lists.xensource.com/xen-devel
>> _______________________________________________
>> Xen-devel mailing list
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>> http://lists.xensource.com/xen-devel
> >

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