From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753530AbYIXSEW (ORCPT ); Wed, 24 Sep 2008 14:04:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751602AbYIXSEN (ORCPT ); Wed, 24 Sep 2008 14:04:13 -0400 Received: from smtp-out.google.com ([216.239.33.17]:29038 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751436AbYIXSEM (ORCPT ); Wed, 24 Sep 2008 14:04:12 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=message-id:date:from:to:subject:cc:in-reply-to: mime-version:content-type:content-transfer-encoding: content-disposition:references:x-gmailtapped-by; b=dwPlm/axu3iEvPwVMNS1L8ndCYV0nvnFpVQkkqVwyvGwNCw3w1C4EVmQRbgoW4Av3 honUc/TSRY6ZAn16Ow2kA== Message-ID: <33307c790809241104k2af4b900w781aba1a1393004b@mail.gmail.com> Date: Wed, 24 Sep 2008 11:04:04 -0700 From: "Martin Bligh" To: "Linus Torvalds" Subject: Re: [RFC PATCH 1/3] Unified trace buffer Cc: "Peter Zijlstra" , "Steven Rostedt" , linux-kernel@vger.kernel.org, "Ingo Molnar" , "Thomas Gleixner" , "Andrew Morton" , prasad@linux.vnet.ibm.com, "Mathieu Desnoyers" , "Frank Ch. Eigler" , "David Wilder" , hch@lst.de, "Tom Zanussi" , "Steven Rostedt" In-Reply-To: <33307c790809241054n47addd27hef90930f109599fd@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <20080924051056.650388887@goodmis.org> <20080924051400.195780424@goodmis.org> <1222268595.16700.149.camel@lappy.programming.kicks-ass.net> <33307c790809240847r31c8b683na15ff5488b60d25b@mail.gmail.com> <1222272686.16700.162.camel@lappy.programming.kicks-ass.net> <33307c790809240949i3026170i8f9ac1d67a0fcf00@mail.gmail.com> <33307c790809241054n47addd27hef90930f109599fd@mail.gmail.com> X-GMailtapped-By: 172.28.16.76 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > If we just record the TSC unshifted, in 27 bits, at 4GHz, that gives us > about 1/30 of a second? So we either shift, use > 27 bits, or record > at least 30 events a second, none of which I like much ... If we use 32 bits instead of 27, then the timestamp events are only about once per second, which is probably fine for overhead ... ? I think we're OK losing 5 bits of precision, that's only 32 cycles, given all the CPU reordering stuff we've talked about here, not-quite-synced TSCs, etc. I suspect you thought we were shifting by much more than this, in reality it was 5-10 bits, with timestamp events inbetween, though we put wall time in them, which I think was a mistake. (note: I'm not suggesting we have to use this compact a format, at least by default)