Messages in this thread | | | Date | Tue, 30 Oct 2012 11:27:52 +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 10/29/2012 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..
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