From mboxrd@z Thu Jan 1 00:00:00 1970 From: George Dunlap Subject: RE: [PATCH] scheduler rate controller Date: Fri, 28 Oct 2011 17:18:34 +0100 Message-ID: <1319818714.21033.414.camel@elijah> References: <1319789425.19320.12.camel@Abyss> <1319796584.19320.31.camel@Abyss> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1319796584.19320.31.camel@Abyss> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dario Faggioli Cc: "Tian, Kevin" , "xen-devel@lists.xensource.com" , "Keir (Xen.org)" , George Dunlap , "Dong, Eddie" , "Lv, Hui" , "Duan, Jiangang" List-Id: xen-devel@lists.xenproject.org On Fri, 2011-10-28 at 11:09 +0100, Dario Faggioli wrote: > Not sure yet, I can imagine it's tricky and I need to dig a bit more in > the code, but I'll let know if I found a way of doing that... There are lots of reasons why the SCHEDULE_SOFTIRQ gets raised. But I think we want to focus on the scheduler itself raising it as a result of the .wake() callback. Whether the .wake() happens as a result of a HW interrupt or something else, I don't think really matters. Dario and Hui, neither of you have commented on my idea, which is simply don't preempt a VM if it has run for less than some amount of time (say, 500us or 1ms). If a higher-priority VM is woken up, see how long the current VM has run. If it's less than 1ms, set a 1ms timer and call schedule() then. > > > More generally speaking, I see how this feature can be useful, and I > > > also think it could live in the generic schedule.c code, but (as George > > > was saying) the algorithm by which rate-limiting is happening needs to > > > be well known, documented and exposed to the user (more than by means > > > of a couple of perf-counters). > > > > > > > One question is that, what is the right palace to document such information? I'd like to make it as clear as possible to the users. > > > Well, don't know, maybe a WARN (a WARN_ONCE alike thing would probably > be better), or in general something that leave a footstep in the logs, > so that one can find out by means of `xl dmesg' or related. Obviously, > I'm not suggesting of printk-ing each suppressed schedule invocation, or > the overhead would get even worse... :-P > > I'm thinking of something that happens the very first time the limiting > fires, or maybe oncee some period/number of suppressions, just to remind > the user that he's getting weird behaviour because _he_enabled_ > rate-limiting. Hopefully, that might also be useful for the user itself > to fine tune the limiting parameters, although I think the perf-counters > are already quite well suited for this. As much as possible, we want the system to Just Work. Under normal circumstances it wouldn't be too unusual for a VM to have a several-ms delay between receiving a physical interrupt and being scheduled; I think that if the 1ms delay works, having it on all the time would probably be the best solution. That's another reason I'm in favor of trying it -- it's simple and easy to understand, and doesn't require detecting when to "turn it on". -George