Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle differentcommit cases accordingly
From: Raghavendra K T
Date: Tue Oct 30 2012 - 03:37:10 EST
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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