Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754230Ab0DYOPl (ORCPT ); Sun, 25 Apr 2010 10:15:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:27185 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752413Ab0DYOPk (ORCPT ); Sun, 25 Apr 2010 10:15:40 -0400 Message-ID: <4BD44E74.2020506@redhat.com> Date: Sun, 25 Apr 2010 17:15:16 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Dan Magenheimer CC: linux-kernel@vger.kernel.org, linux-mm@kvack.org, jeremy@goop.org, hugh.dickins@tiscali.co.uk, ngupta@vflare.org, JBeulich@novell.com, chris.mason@oracle.com, kurt.hackel@oracle.com, dave.mccracken@oracle.com, npiggin@suse.de, akpm@linux-foundation.org, riel@redhat.com Subject: Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview References: <20100422134249.GA2963@ca-server1.us.oracle.com> <4BD06B31.9050306@redhat.com> <53c81c97-b30f-4081-91a1-7cef1879c6fa@default> <4BD07594.9080905@redhat.com> <4BD16D09.2030803@redhat.com> <4BD1A74A.2050003@redhat.com> <4830bd20-77b7-46c8-994b-8b4fa9a79d27@default> <4BD1B427.9010905@redhat.com> <4BD336CF.1000103@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2131 Lines: 47 On 04/25/2010 04:37 PM, Dan Magenheimer wrote: >> My issue is with the API's synchronous nature. Both RAM and more >> exotic >> memories can be used with DMA instead of copying. A synchronous >> interface gives this up. >> : >> Let's not allow the urge to merge prevent us from doing the right >> thing. >> : >> I see. Given that swap-to-flash will soon be way more common than >> frontswap, it needs to be solved (either in flash or in the swap code). >> > While I admit that I started this whole discussion by implying > that frontswap (and cleancache) might be useful for SSDs, I think > we are going far astray here. Frontswap is synchronous for a > reason: It uses real RAM, but RAM that is not directly addressable > by a (guest) kernel. SSD's (at least today) are still I/O devices; > even though they may be very fast, they still live on a PCI (or > slower) bus and use DMA. Frontswap is not intended for use with > I/O devices. > > Today's memory technologies are either RAM that can be addressed > by the kernel, or I/O devices that sit on an I/O bus. The > exotic memories that I am referring to may be a hybrid: > memory that is fast enough to live on a QPI/hypertransport, > but slow enough that you wouldn't want to randomly mix and > hand out to userland apps some pages from "exotic RAM" and some > pages from "normal RAM". Such memory makes no sense today > because OS's wouldn't know what to do with it. But it MAY > make sense with frontswap (and cleancache). > > Nevertheless, frontswap works great today with a bare-metal > hypervisor. I think it stands on its own merits, regardless > of one's vision of future SSD/memory technologies. > Even when frontswapping to RAM on a bare metal hypervisor it makes sense to use an async API, in case you have a DMA engine on board. -- error compiling committee.c: too many arguments to function -- 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/