Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752235AbZGTLQe (ORCPT ); Mon, 20 Jul 2009 07:16:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751257AbZGTLQc (ORCPT ); Mon, 20 Jul 2009 07:16:32 -0400 Received: from casper.infradead.org ([85.118.1.10]:55124 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751009AbZGTLQb (ORCPT ); Mon, 20 Jul 2009 07:16:31 -0400 Subject: Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE From: Peter Zijlstra To: Arjan van de Ven Cc: john stultz , Thomas Gleixner , lkml , Thomas Gleixner , Ingo Molnar , Andi Kleen , nikolag@ca.ibm.com, Darren Hart In-Reply-To: <20090718153011.1de3af8e@infradead.org> References: <1247873945.8334.67.camel@localhost.localdomain> <1247954978.14494.19.camel@work-vm> <20090718153011.1de3af8e@infradead.org> Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Mon, 20 Jul 2009 13:17:02 +0200 Message-Id: <1248088622.15751.8465.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2001 Lines: 42 On Sat, 2009-07-18 at 15:30 -0700, Arjan van de Ven wrote: > On Sat, 18 Jul 2009 15:09:38 -0700 > john stultz wrote: > > > After talking with some application writers who want very fast, but > > not fine-grained timestamps, I decided to try to implement a new > > clock_ids to clock_gettime(): CLOCK_REALTIME_COARSE and > > CLOCK_MONOTONIC_COARSE which returns the time at the last tick. This > > is very fast as we don't have to access any hardware (which can be > > very painful if you're using something like the acpi_pm clocksource), > > and we can even use the vdso clock_gettime() method to avoid the > > syscall. The only trade off is you only get low-res tick grained time > > resolution. > > Does this tie us to having a tick? I still have hope that we can get > rid of the tick even when apps are running .... since with CFS we don't > really need the tick for the scheduler anymore for example.... On the hardware side to make this happen we'd need a platform that has: - cheap, high-res, cross-cpu synced, clocksource - cheap, high-res, clockevents Maybe power64, sparc64 and s390x qualify, but certainly nothing on x86 does. Furthermore, on the software side we'd need a few modifications, such as doing lazy accounting for things like u/s-time which currently rely on the tick and moving the load-balancing into a hrtimer. Also, even with the above done, we'd probably want to tinker with the clockevent/hrtimer code and possibly use a second per-cpu hardware timer for the scheduler, since doing the whole hrtimer rb-tree dance for every context switch is simply way too expensive. But even with all that manged, there's still other bits that rely on the tick -- RCU being one of the more interesting ones. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/