linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

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Date:	Tue, 25 Sep 2012 17:00:30 +0200
From:	Dor Laor <dlaor@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Chegu Vinod <chegu_vinod@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios
 in PLE handler

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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