From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id 1E08B6B01E3 for ; Sun, 25 Apr 2010 08:16:32 -0400 (EDT) Message-ID: <4BD4329A.9010509@redhat.com> Date: Sun, 25 Apr 2010 15:16:26 +0300 From: Avi Kivity 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> <4BD3B2D1.8080203@vflare.org> In-Reply-To: <4BD3B2D1.8080203@vflare.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: ngupta@vflare.org 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 04/25/2010 06:11 AM, Nitin Gupta wrote: > 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). > Doesn't flash have similar requirements? The earlier you discard, the likelier you are to reuse an erase block (or reduce the amount of copying). > 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. > That's fine, swap works under low memory conditions by using reserves. > - 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). > How does frontswap do this? Does it maintain its own data structures? >> 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. > I'm not sure. Swap-to-flash will soon be everywhere. If it's slow, people will feel it a lot more than ramzswap slowness. > 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. > Even for copying to RAM an async API is wanted, so you can dma it instead of copying. -- error compiling committee.c: too many arguments to function -- 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