From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_NONE autolearn=ham autolearn_force=no version=3.4.2 Received: from minnie.tuhs.org (minnie.tuhs.org [45.79.103.53]) by inbox.vuxu.org (OpenSMTPD) with ESMTP id 7470dbdf for ; Tue, 27 Aug 2019 14:56:30 +0000 (UTC) Received: by minnie.tuhs.org (Postfix, from userid 112) id 405AB9BC04; Wed, 28 Aug 2019 00:56:29 +1000 (AEST) Received: from minnie.tuhs.org (localhost [127.0.0.1]) by minnie.tuhs.org (Postfix) with ESMTP id DC9AE9B684; Wed, 28 Aug 2019 00:56:00 +1000 (AEST) Received: by minnie.tuhs.org (Postfix, from userid 112) id 0E69A9BB7B; Wed, 28 Aug 2019 00:55:58 +1000 (AEST) Received: from mcvoy.com (mcvoy.com [192.169.23.250]) by minnie.tuhs.org (Postfix) with ESMTPS id 0B0F09B684 for ; Wed, 28 Aug 2019 00:55:57 +1000 (AEST) Received: by mcvoy.com (Postfix, from userid 3546) id 8EFCA35E0A8; Tue, 27 Aug 2019 07:55:56 -0700 (PDT) Date: Tue, 27 Aug 2019 07:55:56 -0700 From: Larry McVoy To: Arthur Krewat Message-ID: <20190827145556.GD13570@mcvoy.com> References: <13c5c36e-c84d-e020-d09e-51c8c502dc6d@kilonet.net> <75a32043-4830-ba04-ee0f-023c5f5ade3f@gmail.com> <20190827003013.GS13570@mcvoy.com> <312b39a1-2944-100f-55e0-fc65b504d43d@kilonet.net> <20190827024511.GU13570@mcvoy.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Subject: Re: [TUHS] If not Linux, then what? X-BeenThere: tuhs@minnie.tuhs.org X-Mailman-Version: 2.1.26 Precedence: list List-Id: The Unix Heritage Society mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: TUHS main list Errors-To: tuhs-bounces@minnie.tuhs.org Sender: "TUHS" On Mon, Aug 26, 2019 at 11:14:45PM -0400, Arthur Krewat wrote: > On 8/26/2019 10:45 PM, Larry McVoy wrote: > > Which was that the page cache is > >*the* cache. There is nothing else. > Yeah, I re-read what you wrote a few times after I replied, and realized > what you meant ... eventually ;) I might be making too big of a deal about it. mmap semantics mattered a lot when SMPs first showed up and main memory was small. It meant that you could have multiple CPUs seeing and working on the same chunk of data at the same time. It's very similar to way that IOMMUs are exposed to user space these days, enabling virtual machines direct access to the I/O devices. ZFS breaks that model, the data is all in the ARC and if you mmap it they have to bcopy the data out of the ARC, into the page cache and now they have a consistency problem, you could modify stuff via mmap or write and they have to manage that. That consistency problem is the main reason that Sun almost completely killed the buffer cache (it still was used for inodes and directories but that was it). That consistency problem is a pain in the rear, all sorts of race conditions and it tended to bit rot. Jeff and Bill are smart people so I suspect they got it right but I'm still stunned that they took such an architecturally bad approach. And even more stunned that the oversight people approved it. There is zero chance that the Sun I worked at would have allowed that. --lm