Messages in this thread | | | Date | Tue, 10 Jul 2012 14:56:12 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler |
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On 07/10/2012 03:17 AM, Andrew Theurer wrote: > On Mon, 2012-07-09 at 11:50 +0530, Raghavendra K T wrote: >> Currently Pause Looop Exit (PLE) handler is doing directed yield to a >> random VCPU on PL exit. Though we already have filtering while choosing >> the candidate to yield_to, we can do better. > > Hi, Raghu. Hi Andrew, Thank you for your analysis and inputs
> >> Problem is, for large vcpu guests, we have more probability of yielding >> to a bad vcpu. We are not able to prevent directed yield to same guy who >> has done PL exit recently, who perhaps spins again and wastes CPU. >> >> Fix that by keeping track of who has done PL exit. So The Algorithm in series >> give chance to a VCPU which has: >> >> (a) Not done PLE exit at all (probably he is preempted lock-holder) >> >> (b) VCPU skipped in last iteration because it did PL exit, and probably >> has become eligible now (next eligible lock holder) >> >> Future enhancemnets: >> (1) Currently we have a boolean to decide on eligibility of vcpu. It >> would be nice if I get feedback on guest (>32 vcpu) whether we can >> improve better with integer counter. (with counter = say f(log n )). >> >> (2) We have not considered system load during iteration of vcpu. With >> that information we can limit the scan and also decide whether schedule() >> is better. [ I am able to use #kicked vcpus to decide on this But may >> be there are better ideas like information from global loadavg.] >> >> (3) We can exploit this further with PV patches since it also knows about >> next eligible lock-holder. >> >> Summary: There is a huge improvement for moderate / no overcommit scenario >> for kvm based guest on PLE machine (which is difficult ;) ). >> >> Result: >> Base : kernel 3.5.0-rc5 with Rik's Ple handler fix >> >> Machine : Intel(R) Xeon(R) CPU X7560 @ 2.27GHz, 4 numa node, 256GB RAM, >> 32 core machine > > Is this with HT enabled, therefore 64 CPU threads?
No. HT disabled with 32 online CPUs
> >> Host: enterprise linux gcc version 4.4.6 20120305 (Red Hat 4.4.6-4) (GCC) >> with test kernels >> >> Guest: fedora 16 with 32 vcpus 8GB memory. > > Can you briefly explain the 1x and 2x configs? This of course is highly > dependent whether or not HT is enabled...
1x config: kernbench/ebizzy/sysbench running on 1 guest (32 vcpu) all the benchmarks have 2*#vcpu = 64 threads
2x config: kernbench/ebizzy/sysbench running on 2 guests each with 32 vcpu) all the benchmarks have 2*#vcpu = 64 threads
> > FWIW, I started testing what I would call "0.5x", where I have one 40 > vcpu guest running on a host with 40 cores and 80 CPU threads total (HT > enabled, no extra load on the system). For ebizzy, the results are > quite erratic from run to run, so I am inclined to discard it as a
I will be posting full run detail (individual run) in reply to this mail since it is big. I have posted stdev also with the result.. it has not shown too much deviation.
> workload, but maybe I should try "1x" and "2x" cpu over-commit as well. > >> From initial observations, at least for the ebizzy workload, the > percentage of exits that result in a yield_to() are very low, around 1%, > before these patches.
Hmm Ok.. IMO for a under-committed workload, probably low percentage of yield_to was expected, but not sure whether 1% is too less though. But importantly, number of successful yield_to can never measure benefit.
With this patch what I am trying to address is to ensure successful yield_to result in benefit.
So, I am concerned that at least for this test, > reducing that number even more has diminishing returns. I am however > still concerned about the scalability problem with yield_to(),
So did you mean you are expected to see more yield_to overheads with large guests? As already mentioned in future enhancements, one thing I will be trying in future would be,
a. have counter instead of boolean for skipping yield_to b. just scan probably f(log(n)) vcpu to yield and then schedule()/ return depending on system load.
so we will be reducing overall vcpu iteration in PLE handler from O(n * n) to O(n log n)
which > shows like this for me (perf): > >> 63.56% 282095 qemu-kvm [kernel.kallsyms] [k] _raw_spin_lock >> 5.42% 24420 qemu-kvm [kvm] [k] kvm_vcpu_yield_to >> 5.33% 26481 qemu-kvm [kernel.kallsyms] [k] get_pid_task >> 4.35% 20049 qemu-kvm [kernel.kallsyms] [k] yield_to >> 2.74% 15652 qemu-kvm [kvm] [k] kvm_apic_present >> 1.70% 8657 qemu-kvm [kvm] [k] kvm_vcpu_on_spin >> 1.45% 7889 qemu-kvm [kvm] [k] vcpu_enter_guest > > For the cpu threads in the host that are actually active (in this case > 1/2 of them), ~50% of their time is in kernel and ~43% in guest.This > is for a no-IO workload, so that's just incredible to see so much cpu > wasted. I feel that 2 important areas to tackle are a more scalable > yield_to() and reducing the number of pause exits itself (hopefully by > just tuning ple_window for the latter).
I think this is a concern and as you stated I agree that tuning ple_window helps here.
> > Honestly, I not confident addressing this problem will improve the > ebizzy score. That workload is so erratic for me, that I do not trust > the results at all. I have however seen consistent improvements in > disabling PLE for a http guest workload and a very high IOPS guest > workload, both with much time spent in host in the double runqueue lock > for yield_to(), so that's why I still gravitate toward that issue.
The problem starts (in PLE disabled) when we have workload just > 1x.We start burning so much of cpu.
IIRC, in 2x overcommit, kernel compilation that takes 10hr on non-PLE, used to take just 1hr after pv patches (and should be same with PLE enabled)
If we leave PLE disabled case, I do not expect any degradation even in 0.5 x scenario, though you say results are erratic.
Could you please let me know, When PLE was enabled, before and after the patch did you see any degradation for 0.5x?
> -Andrew Theurer > >
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