From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] [RFC] Next gen kvm api Date: Tue, 07 Feb 2012 18:02:50 +0200 Message-ID: <4F314B2A.4000709@redhat.com> References: <4F2AB552.2070909@redhat.com> <4F2E80A7.5040908@redhat.com> <4F3025FB.1070802@codemonkey.ws> <4F31132F.3010100@redhat.com> <4F31408F.80901@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Rob Earhart , linux-kernel , KVM list , qemu-devel To: Anthony Liguori Return-path: In-Reply-To: <4F31408F.80901@codemonkey.ws> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.