From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jan Beulich" <JBeulich@suse.com>
Subject: Re: [PATCH v2] xen/credit scheduler;
 Use delay to control scheduling frequency
Date: Mon, 19 Dec 2011 15:38:06 +0000
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To: George Dunlap <George.Dunlap@eu.citrix.com>, Hui Lv <hui.lv@intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, "keir@xen.org" <keir@xen.org>, Eddie Dong <eddie.dong@intel.com>, Jiangang Duan <jiangang.duan@intel.com>, Zhidong Yu <zhidong.yu@intel.com>
List-Id: xen-devel@lists.xenproject.org

>>> On 19.12.11 at 16:25, "Lv, Hui" <hui.lv@intel.com> wrote:
>> Overriding the rate limit by the time slice isn't the right thing either, as 
> that
>> (the way I "read" it) means there's no rate limiting at all.
>> What "rate limit" to me means is preventing quickly switching away from a
>> vCPU recently scheduled without extending its (remaining) time slice, i.e. 
> in any
>> place a respective evaluation is done the shorter of the two should be used.
>> 
>> Jan
> 
> So the basic thing is to avoid "time slice" < "rate limit", happen.
> I really don't understand why people want a 1ms time slice, but set the 
> rate_limit to 5000(us), that is insubstantial.

So if someone set the (global) rate limit value to 5000us and then,
days or weeks later, migrates a VM with a 3ms time slice to that
host, why should this be an admin mistake? To me, the rate limit is a
performance improvement, while the time slice may be (an indirect
result of) a to be enforced policy.

Jan

> If, this unfortunately happens, I prefer to put "rate_limit" at higher 
> priority, which means extending the running time slice.
> Some warnings should be put before the parameter of sched_ratelimit_us to 
> avoid this.
> Is this reasonable?