Messages in this thread | | | Date | Tue, 30 Oct 2012 13:01:54 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle different commit cases accordingly |
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On 10/30/2012 12:04 PM, Andrew Jones wrote: > On Tue, Oct 30, 2012 at 11:27:52AM +0530, Raghavendra K T wrote: >> On 10/29/2012 11:24 PM, Peter Zijlstra wrote: >>> On Mon, 2012-10-29 at 19:37 +0530, Raghavendra K T wrote: >>>> +/* >>>> + * A load of 2048 corresponds to 1:1 overcommit >>>> + * undercommit threshold is half the 1:1 overcommit >>>> + * overcommit threshold is 1.75 times of 1:1 overcommit threshold >>>> + */ >>>> +#define COMMIT_THRESHOLD (FIXED_1) >>>> +#define UNDERCOMMIT_THRESHOLD (COMMIT_THRESHOLD >> 1) >>>> +#define OVERCOMMIT_THRESHOLD ((COMMIT_THRESHOLD << 1) - >>>> (COMMIT_THRESHOLD >> 2)) >>>> + >>>> +unsigned long kvm_system_load(void) >>>> +{ >>>> + unsigned long load; >>>> + >>>> + load = avenrun[0] + FIXED_1/200; >>>> + load = load / num_online_cpus(); >>>> + >>>> + return load; >>>> +} >>> >>> ARGH.. no that's wrong.. very wrong. >>> >>> 1) avenrun[] EXPORT_SYMBOL says it should be removed, that's not a >>> joke. >> >> Okay. >> >>> 2) avenrun[] is a global load, do not ever use a global load measure >> >> This makes sense. Using a local optimization that leads to near global >> optimization is the way to go. >> >>> >>> 3) avenrun[] has nothing what so ever to do with runqueue lengths, >>> someone with a gazillion tasks in D state will get a huge load but the >>> cpu is very idle. >>> >> >> I used loadavg as an alternative measure. But the above condition >> poses a concern for that. >> >> Okay, now IIUC, usage of *any* global measure is bad? >> >> Because I was also thinking to use nrrunning()/ num_online_cpus(), to >> get an idea of global overcommit sense. (ofcourse since, this involves >> iteration over per CPU nrrunning, I wanted to calculate this >> periodically) >> >> The overall logic, of having overcommit_threshold, >> undercommit_threshold, I wanted to use for even dynamic ple_window >> tuning purpose. >> >> so logic was: >> < undercommit_threshold => 16k ple_window >>> overcommit_threshold => 4k window. >> for in between case scale the ple_window accordingly. >> >> The alternative was to decide depending on how ple handler succeeded in >> yield_to. But I thought, that is too sensitive and more overhead. >> >> This topic may deserve different thread, but thought I shall table it here. >> >> So, Thinking about the alternatives to implement, logic such as >> >> (a) if(undercommitted) >> just go back and spin rather than going for yield_to iteration. >> (b) if (overcommitted) >> better to yield rather than spinning logic >> >> of current patches.. >> >> [ ofcourse, (a) is already met to large extent by your patches..] >> >> So I think everything boils down to >> >> "how do we measure these two thresholds without much overhead in a >> compliant way" >> >> Ideas welcome.. >> > > What happened to Avi's preempt notifier idea for determining > under/overcommit? If nobody has picked that up yet, then I'll go ahead and > try to prototype it.
Hi Drew,
I had assumed my priority order as 1) this patch series 2) dynamic ple window 3) preempt notifiers.
But I do not have any problem on re-prioritizing / helping on these as far as we are clear on what we are looking into.
I was thinking about preempt notifier idea as a tool to refine candidate VCPUs. But you are right, Avi, also told we can use bitmap/counter itself as an indicator to decide whether we go ahead with yield_to at all.
IMO, only patch(3) has some conflict because of various approach we can try.May be we should attack the problem via all 3 solutions at once and decide?
To be frank, within each of the approach, trying/analyzing all the possibilities made the things slow.. (my end).
Suggestions..?
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