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

[Dor Laor]

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.

Thanks for following up!
Dor
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