Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262966AbUDAQbg (ORCPT ); Thu, 1 Apr 2004 11:31:36 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262471AbUDAQbY (ORCPT ); Thu, 1 Apr 2004 11:31:24 -0500 Received: from mail.shareable.org ([81.29.64.88]:14229 "EHLO mail.shareable.org") by vger.kernel.org with ESMTP id S262260AbUDAQbS (ORCPT ); Thu, 1 Apr 2004 11:31:18 -0500 Date: Thu, 1 Apr 2004 17:30:47 +0100 From: Jamie Lokier To: Arjan van de Ven Cc: Albert Cahalan , "Randy.Dunlap" , Peter Williams , ak@muc.de, Richard.Curnow@superh.com, aeb@cwi.nl, linux-kernel mailing list Subject: Re: finding out the value of HZ from userspace Message-ID: <20040401163047.GD25502@mail.shareable.org> References: <1079453698.2255.661.camel@cube> <20040320095627.GC2803@devserv.devel.redhat.com> <1079794457.2255.745.camel@cube> <405CDA9C.6090109@aurema.com> <20040331134009.76ca3b6d.rddunlap@osdl.org> <1080776817.2233.2326.camel@cube> <20040401155420.GB25502@mail.shareable.org> <20040401160132.GB13294@devserv.devel.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20040401160132.GB13294@devserv.devel.redhat.com> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1210 Lines: 28 Arjan van de Ven wrote: > HZ doesn't mean nothing, esp when we go to a tickless kernel... As explained several times in this thread, HZ is meaningful because it affects the rounding in select/poll/epoll/setitimer. A few userspace programs with low jitter soft-RT timing requirements need to compensate for that rounding and/or deliberately synchronise themselves with the tick. Such programs can determine HZ experimentally and lock onto the tick in the manner of a PLL, but it would be nice to simply be able to have the value, to reduce the number of control variables. When we go to a tickless kernel and offer high-resolution timers to userspace, then it will be irrelevant. Until then, or if the kernel goes tickless but limits the resolution of timers for efficiency, the value of HZ is still relevant. Not to get irritatingly back to the subject of this thread or anything, but... is the value of HZ reported to userspace anywhere? Thanks :) -- Jamie - 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/