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

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Message-ID: <50604BF0.1070607@linux.vnet.ibm.com>
Date:	Mon, 24 Sep 2012 17:32:56 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	dlaor@...hat.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 02:12 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?
Also I believe time should be passed via sysfs / hardcoded for each
type of lock we are mimicking

>
> 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.

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