From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 84FEF6B0234 for ; Sat, 24 Apr 2010 23:14:16 -0400 (EDT) Received: by pvf33 with SMTP id 33so368454pvf.14 for ; Sat, 24 Apr 2010 20:14:15 -0700 (PDT) Message-ID: <4BD3B2D1.8080203@vflare.org> Date: Sun, 25 Apr 2010 08:41:13 +0530 From: Nitin Gupta Reply-To: ngupta@vflare.org MIME-Version: 1.0 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> <4830bd20-77b7-46c8-994b-8b4fa9a79d27@default> <4BD1B427.9010905@redhat.com> <4BD24E37.30204@vflare.org> <4BD33822.2000604@redhat.com> In-Reply-To: <4BD33822.2000604@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Avi Kivity Cc: Dan Magenheimer , linux-kernel@vger.kernel.org, linux-mm@kvack.org, jeremy@goop.org, hugh.dickins@tiscali.co.uk, 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 List-ID: On 04/24/2010 11:57 PM, Avi Kivity wrote: > On 04/24/2010 04:49 AM, Nitin Gupta wrote: >> >>> I see. So why not implement this as an ordinary swap device, with a >>> higher priority than the disk device? this way we reuse an API and keep >>> things asynchronous, instead of introducing a special purpose API. >>> >>> >> ramzswap is exactly this: an ordinary swap device which stores every page >> in (compressed) memory and its enabled as highest priority swap. >> Currently, >> it stores these compressed chunks in guest memory itself but it is not >> very >> difficult to send these chunks out to host/hypervisor using virtio. >> >> However, it suffers from unnecessary block I/O layer overhead and >> requires >> weird hooks in swap code, say to get notification when a swap slot is >> freed. >> > > Isn't that TRIM? No: trim or discard is not useful. The problem is that we require a callback _as soon as_ a page (swap slot) is freed. Otherwise, stale data quickly accumulates in memory defeating the whole purpose of in-memory compressed swap devices (like ramzswap). Increasing the frequency of discards is also not an option: - Creating discard bio requests themselves need memory and these swap devices come into picture only under low memory conditions. - We need to regularly scan swap_map to issue these discards. Increasing discard frequency also means more frequent scanning (which will still not be fast enough for ramzswap needs). > >> OTOH frontswap approach gets rid of any such artifacts and overheads. >> (ramzswap: http://code.google.com/p/compcache/) >> > > Maybe we should optimize these overheads instead. Swap used to always > be to slow devices, but swap-to-flash has the potential to make swap act > like an extension of RAM. > Spending lot of effort optimizing an overhead which can be completely avoided is probably not worth it. Also, I think the choice of a synchronous style API for frontswap and cleancache is justified as they want to send pages to host *RAM*. If you want to use other devices like SSDs, then these should be just added as another swap device as we do currently -- these should not be used as frontswap storage directly. Thanks, Nitin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org