linux-kernel - Re: [Qemu-devel] [RFC] Next gen kvm api

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Date:	Tue, 07 Feb 2012 09:17:35 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Avi Kivity <avi@...hat.com>
CC:	Rob Earhart <earhart@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

On 02/07/2012 06:03 AM, Avi Kivity wrote:
> On 02/06/2012 09:11 PM, Anthony Liguori wrote:
>>
>> I'm not so sure. ioeventfds and a future mmio-over-socketpair have to put the
>> kthread to sleep while it waits for the other end to process it. This is
>> effectively equivalent to a heavy weight exit. The difference in cost is
>> dropping to userspace which is really neglible these days (< 100 cycles).
>
> On what machine did you measure these wonderful numbers?

A syscall is what I mean by "dropping to userspace", not the cost of a heavy 
weight exit.  I think a heavy weight exit is still around a few thousand cycles.

Any nehalem class or better processor should have a syscall cost of around that 
unless I'm wildly mistaken.

>
> But I agree a heavyweight exit is probably faster than a double context switch
> on a remote core.

I meant, if you already need to take a heavyweight exit (and you do to schedule 
something else on the core), than the only additional cost is taking a syscall 
return to userspace *first* before scheduling another process.  That overhead is 
pretty low.

Regards,

Anthony Liguori

>
>

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