Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

[Andrew Theurer]

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.

> 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?

> 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...

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
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.  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(), 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).

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.


-Andrew Theurer

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