From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262325AbVHCQca (ORCPT ); Wed, 3 Aug 2005 12:32:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262324AbVHCQc3 (ORCPT ); Wed, 3 Aug 2005 12:32:29 -0400 Received: from atlrel6.hp.com ([156.153.255.205]:387 "EHLO atlrel6.hp.com") by vger.kernel.org with ESMTP id S262325AbVHCQcW (ORCPT ); Wed, 3 Aug 2005 12:32:22 -0400 Subject: Re: [PATCH] optimize writer path in time_interpolator_get_counter() From: Alex Williamson To: Christoph Lameter Cc: tony.luck@intel.com, linux-kernel@vger.kernel.org, akpm@osdl.org In-Reply-To: References: <1122911571.5243.23.camel@tdi> <200508021837.j72Ib7T9020681@agluck-lia64.sc.intel.com> <1123080155.5193.15.camel@tdi> Content-Type: text/plain Organization: LOSL Date: Wed, 03 Aug 2005 10:32:13 -0600 Message-Id: <1123086733.5193.69.camel@tdi> Mime-Version: 1.0 X-Mailer: Evolution 2.2.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2005-08-03 at 09:10 -0700, Christoph Lameter wrote: > On Wed, 3 Aug 2005, Alex Williamson wrote: > > > be a reasonable performance vs absolute accuracy trade-off. What > > happens to your worst case time if you (just for a test) hard code a > > min_delta of something around 20-50? There could be some kind of > > Think about a threaded process that gets time on multiple processors > and then compares the times. This means that the time value obtained later > on one thread may indicate a time earlier than that obtained on another > thread. An essential requirement for time values is that they are > monotonically increasing. You are changing that basic nature. Ok, I can see the scenario where that could produce jitter. However, that implies than any exit through that path could produce jitter as it is. For instance: CPU0 CPU1 read lcycle read itc read lcycle read itc cmpxchg oops, lcycle is stale So the window already exists for this... Alex -- Alex Williamson HP Linux & Open Source Lab