From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [RFC] Next gen kvm api Date: Tue, 07 Feb 2012 17:18:54 +0100 Message-ID: <4F314EEE.8080401@siemens.com> References: <4F2AB552.2070909@redhat.com> <4F2E80A7.5040908@redhat.com> <4F3025FB.1070802@codemonkey.ws> <4F31132F.3010100@redhat.com> <4F31408F.80901@codemonkey.ws> <4F314B2A.4000709@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: qemu-devel , Rob Earhart , linux-kernel , KVM list To: Avi Kivity Return-path: In-Reply-To: <4F314B2A.4000709@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org List-Id: kvm.vger.kernel.org On 2012-02-07 17:02, Avi Kivity wrote: > On 02/07/2012 05:17 PM, Anthony Liguori wrote: >> 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. > > Ah. But then ioeventfd has that as well, unless the other end is in the > kernel too. > >> 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. >> > > That's what I remember too. > >>> >>> 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. > > Yeah. > Isn't there another level in between just scheduling and full syscall return if the user return notifier has some real work to do? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux