From owner-freebsd-current@FreeBSD.ORG Fri Oct 28 13:21:04 2005 Return-Path: X-Original-To: current@freebsd.org Delivered-To: freebsd-current@FreeBSD.ORG Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id CB62C16A41F; Fri, 28 Oct 2005 13:21:04 +0000 (GMT) (envelope-from phk@critter.freebsd.dk) Received: from phk.freebsd.dk (phk.freebsd.dk [130.225.244.222]) by mx1.FreeBSD.org (Postfix) with ESMTP id EA42143D79; Fri, 28 Oct 2005 13:20:49 +0000 (GMT) (envelope-from phk@critter.freebsd.dk) Received: from critter.freebsd.dk (unknown [192.168.48.2]) by phk.freebsd.dk (Postfix) with ESMTP id 04A57BC84; Fri, 28 Oct 2005 13:20:46 +0000 (UTC) To: Robert Watson From: "Poul-Henning Kamp" In-Reply-To: Your message of "Fri, 28 Oct 2005 14:08:01 BST." <20051028140556.W20147@fledge.watson.org> Date: Fri, 28 Oct 2005 15:20:46 +0200 Message-ID: <32412.1130505646@critter.freebsd.dk> Sender: phk@critter.freebsd.dk Cc: Pertti Kosunen , David Xu , "Yuriy N. Shkandybin" , current@freebsd.org Subject: Re: Timers and timing, was: MySQL Performance 6.0rc1 X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 28 Oct 2005 13:21:04 -0000 In message <20051028140556.W20147@fledge.watson.org>, Robert Watson writes: > >On Fri, 28 Oct 2005, David Xu wrote: > >> Poul-Henning Kamp wrote: >>> In message <4361FDBE.7000500@freebsd.org>, David Xu writes: >>> >>> the correct way to optimize this would be to add a time(2) systemcall >>> which returns the value of the kernel global time_second. >> >> Can we make a page in kernel address space which is readable my user >> code? put the variable in the page, I know read an integer is atomic-op, >> needn't lock, so syscall is not needed. > >This approach has a lot of merit, as we can also potentially export other >information there (such as kernel preferences for system call mechanisms). Yes, there are many advantages to this approach, but we need a solution to the API versioning problem before we head that way. For anyone wanting to look at this, three are a number of nasties to remember: 1. How does userland get hold of the page ? Does it open a magic device ? Use a magic syscall ? Or does all processes just get the page by default ? 2. Where in the address space do we put it ? 3. Layout and alignment issues. Remember that things change size over time. (Version numbers for each element ?) And that cross- arch support is desirable (32bit i386 binaries on 64bit amd64 arch) 4. Do guarantee a syscall fallback for all facilities if there is version skew, or do we abort the program ? 5. Do we want a global system page and a per process page while we are at it. There is plenty of stuff we could put in the per-proc page: pid, ppid, resource usage, proctitle etc. >On the other hand, a lower risk change might be to simply add a new CLOCK_ >type for lower resolution, and have a timer synchronize a variable to the >system clock once every 1/10 of a second. This avoids having to muck with >VM layout, etc. Is the CLOCK_* namespace ours to muck about with in the first place ? -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.