linux-kernel - Re: [PATCH] use unfair spinlock when running on hypervisor.

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Message-ID: <4C053ACC.5020708@redhat.com>
Date:	Tue, 01 Jun 2010 19:52:28 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, hpa@...or.com, mingo@...e.hu, npiggin@...e.de,
	tglx@...utronix.de, mtosatti@...hat.com
Subject: Re: [PATCH] use unfair spinlock when running on hypervisor.

On 06/01/2010 07:38 PM, Andi Kleen wrote:
>>> Your new code would starve again, right?
>>>
>>>        
>> Yes, of course it may starve with unfair spinlock. Since vcpus are not
>> always running there is much smaller chance then vcpu on remote memory
>> node will starve forever. Old kernels with unfair spinlocks are running
>> fine in VMs on NUMA machines with various loads.
>>      
> Try it on a NUMA system with unfair memory.
>    

We are running everything on NUMA (since all modern machines are now 
NUMA).  At what scale do the issues become observable?

>> I understand that reason and do not propose to get back to old spinlock
>> on physical HW! But with virtualization performance hit is unbearable.
>>      
> Extreme unfairness can be unbearable too.
>    

Well, the question is what happens first.  In our experience, vcpu 
overcommit is a lot more painful.  People will never see the NUMA 
unfairness issue if they can't use kvm due to the vcpu overcommit problem.

What I'd like to see eventually is a short-term-unfair, long-term-fair 
spinlock.  Might make sense for bare metal as well.  But it won't be 
easy to write.

-- 
error compiling committee.c: too many arguments to function

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