Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenariosin PLE handler

[Raghavendra K T]

On 09/25/2012 08:30 PM, Dor Laor wrote:
> On 09/24/2012 02:02 PM, Raghavendra K T wrote:
> > On 09/24/2012 02:12 PM, Dor Laor wrote:
> > > In order to help PLE and pvticketlock converge I thought that a small
> > > test code should be developed to test this in a predictable,
> > > deterministic way.
> > >
> > > The idea is to have a guest kernel module that spawn a new thread each
> > > time you write to a /sys/.... entry.
> > >
> > > Each such a thread spins over a spin lock. The specific spin lock is
> > > also chosen by the /sys/ interface. Let's say we have an array of spin
> > > locks *10 times the amount of vcpus.
> > >
> > > All the threads are running a
> > > while (1) {
> > >
> > > spin_lock(my_lock);
> > > sum += execute_dummy_cpu_computation(time);
> > > spin_unlock(my_lock);
> > >
> > > if (sys_tells_thread_to_die()) break;
> > > }
> > >
> > > print_result(sum);
> > >
> > > Instead of calling the kernel's spin_lock functions, clone them and make
> > > the ticket lock order deterministic and known (like a linear walk of all
> > > the threads trying to catch that lock).
> >
> > By Cloning you mean hierarchy of the locks?
>
> No, I meant to clone the implementation of the current spin lock code in
> order to set any order you may like for the ticket selection.
> (even for a non pvticket lock version)
>
> For instance, let's say you have N threads trying to grab the lock, you
> can always make the ticket go linearly from 1->2...->N.
> Not sure it's a good idea, just a recommendation.
>
> > Also I believe time should be passed via sysfs / hardcoded for each
> > type of lock we are mimicking
>
> Yap
>
> > > This way you can easy calculate:
> > > 1. the score of a single vcpu running a single thread
> > > 2. the score of sum of all thread scores when #thread==#vcpu all
> > > taking the same spin lock. The overall sum should be close as
> > > possible to #1.
> > > 3. Like #2 but #threads > #vcpus and other versions of #total vcpus
> > > (belonging to all VMs) > #pcpus.
> > > 4. Create #thread == #vcpus but let each thread have it's own spin
> > > lock
> > > 5. Like 4 + 2
> > >
> > > Hopefully this way will allows you to judge and evaluate the exact
> > > overhead of scheduling VMs and threads since you have the ideal result
> > > in hand and you know what the threads are doing.
> > >
> > > My 2 cents, Dor
> >
> > Thank you,
> > I think this is an excellent idea. ( Though I am trying to put all the
> > pieces together you mentioned). So overall we should be able to measure
> > the performance of pvspinlock/PLE improvements with a deterministic
> > load in guest.
> >
> > Only thing I am missing is,
> > How to generate different combinations of the lock.
> >
> > Okay, let me see if I can come with a solid model for this.
>
> Do you mean the various options for PLE/pvticket/other? I haven't
> thought of it and assumed its static but it can also be controlled
> through the temporary /sys interface.

No, I am not there yet.

So In summary, we are suffering with inconsistent benchmark result,
while measuring the benefit of our improvement in PLE/pvlock etc..

So good point from your suggestion is,
- Giving predictability to workload that runs in guest, so that we have
pi-pi comparison of improvement.

- we can easily tune the workload via sysfs, and we can have script to
automate them.

What is complicated is:
- How can we simulate a workload close to what we measure with
benchmarks?
- How can we mimic lock holding time/ lock hierarchy close to the way
it is seen with real workloads (for e.g. highly contended zone lru lock
with similar amount of lockholding times).
- How close it would be to when we forget about other types of spinning
(for e.g, flush_tlb).

So I feel it is not as trivial as it looks like.

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