From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC] Moving the kvm ioapic, pic, and pit back to userspace Date: Mon, 07 Jun 2010 22:16:55 +0300 Message-ID: <4C0D45A7.8070000@redhat.com> References: <4C0D0FB7.80709@redhat.com> <4C0D1EFA.70104@cisco.com> <4C0D3EA3.1010205@redhat.com> <4C0D4058.3060009@cisco.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: KVM list , qemu-devel To: "David S. Ahern" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:15705 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752103Ab0FGTRF (ORCPT ); Mon, 7 Jun 2010 15:17:05 -0400 In-Reply-To: <4C0D4058.3060009@cisco.com> Sender: kvm-owner@vger.kernel.org List-ID: On 06/07/2010 09:54 PM, David S. Ahern wrote: > >> So it's important to know how often your RHEL3/4 guest queries the PIT >> (not just receives interrupts, actually reads the counter) under a >> realistic load. If you have such a number (in reads/sec) that would be >> a good input to this discussion. >> >> > Aps that invoke gettimeofday a lot. Ask a stupid question, get an "it depends on the workload" answer. > As I recall RHEL3 uses the TSC > between timer interrupts, but RHEL4 samples counters on each > gettimeofday call: > > http://www.mail-archive.com/kvm@vger.kernel.org/msg07231.html > > Because of that performance of applications that timestamp log entries > (like a certain product I work on) takes a hit on KVM unless the TSC is > the clock source. > So it looks like dropping the PIT out of the kernel, let alone the PIC/IOAPIC, is out of the question. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.