From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D1B89C76196 for ; Fri, 31 Mar 2023 07:57:03 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pi9cN-0000Na-Uy; Fri, 31 Mar 2023 03:56:15 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pi9cL-0000NR-MV for qemu-devel@nongnu.org; Fri, 31 Mar 2023 03:56:13 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pi9cJ-0001dD-WB for qemu-devel@nongnu.org; Fri, 31 Mar 2023 03:56:13 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1680249371; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=l8zstMGp7fT4YRW+9qOnRsPwmr7yw/vLfe/5T33B1Qs=; b=Yb0oc8WOviS0gQYS2eI8nhl8Ig35Az1VzAqShqBIvRLSiKUZqh1CkqlmEI+9hDxKwLkYYE CyFHbHfzh8Dyyk5WIufMNFjcVxbeLLh8fHIRegzWPzF9NVuCWhj4pbFEhEhkxww0KoV/tz 1AhIxGwKVMTOpk+cvpqo25/sOZvmUwY= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-576-hca9IofIPzGj6acWv8g_FA-1; Fri, 31 Mar 2023 03:56:07 -0400 X-MC-Unique: hca9IofIPzGj6acWv8g_FA-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E02CF88B7B1; Fri, 31 Mar 2023 07:56:06 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.67]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7D37A492B00; Fri, 31 Mar 2023 07:56:04 +0000 (UTC) Date: Fri, 31 Mar 2023 08:56:01 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Fabiano Rosas , qemu-devel@nongnu.org, Claudio Fontana , jfehlig@suse.com, dfaggioli@suse.com, dgilbert@redhat.com, Juan Quintela , Nikolay Borisov , Paolo Bonzini , David Hildenbrand , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Eric Blake , Markus Armbruster Subject: Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability Message-ID: References: <20230330180336.2791-1-farosas@suse.de> <20230330180336.2791-11-farosas@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Mar 30, 2023 at 06:01:51PM -0400, Peter Xu wrote: > On Thu, Mar 30, 2023 at 03:03:20PM -0300, Fabiano Rosas wrote: > > From: Nikolay Borisov > > > > Implement 'fixed-ram' feature. The core of the feature is to ensure that > > each ram page of the migration stream has a specific offset in the > > resulting migration stream. The reason why we'd want such behavior are > > two fold: > > > > - When doing a 'fixed-ram' migration the resulting file will have a > > bounded size, since pages which are dirtied multiple times will > > always go to a fixed location in the file, rather than constantly > > being added to a sequential stream. This eliminates cases where a vm > > with, say, 1G of ram can result in a migration file that's 10s of > > GBs, provided that the workload constantly redirties memory. > > > > - It paves the way to implement DIO-enabled save/restore of the > > migration stream as the pages are ensured to be written at aligned > > offsets. > > > > The feature requires changing the stream format. First, a bitmap is > > introduced which tracks which pages have been written (i.e are > > dirtied) during migration and subsequently it's being written in the > > resulting file, again at a fixed location for every ramblock. Zero > > pages are ignored as they'd be zero in the destination migration as > > well. With the changed format data would look like the following: > > > > |name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages| > > What happens with huge pages? Would page size matter here? > > I would assume it's fine it uses a constant (small) page size, assuming > that should match with the granule that qemu tracks dirty (which IIUC is > the host page size not guest's). > > But I didn't yet pay any further thoughts on that, maybe it would be > worthwhile in all cases to record page sizes here to be explicit or the > meaning of bitmap may not be clear (and then the bitmap_size will be a > field just for sanity check too). I think recording the page sizes is an anti-feature in this case. The migration format / state needs to reflect the guest ABI, but we need to be free to have different backend config behind that either side of the save/restore. IOW, if I start a QEMU with 2 GB of RAM, I should be free to use small pages initially and after restore use 2 x 1 GB hugepages, or vica-verca. The important thing with the pages that are saved into the file is that they are a 1:1 mapping guest RAM regions to file offsets. IOW, the 2 GB of guest RAM is always a contiguous 2 GB region in the file. If the src VM used 1 GB pages, we would be writing a full 2 GB of data assuming both pages were dirty. If the src VM used 4k pages, we would be writing some subset of the 2 GB of data, and the rest would be unwritten. Either way, when reading back the data we restore it into either 1 GB pages of 4k pages, beause any places there were unwritten orignally will read back as zeros. > If postcopy might be an option, we'd want the page size to be the host page > size because then looking up the bitmap will be straightforward, deciding > whether we should copy over page (UFFDIO_COPY) or fill in with zeros > (UFFDIO_ZEROPAGE). This format is only intended for the case where we are migrating to a random-access medium, aka a file, because the fixed RAM mappings to disk mean that we need to seek back to the original location to re-write pages that get dirtied. It isn't suitable for a live migration stream, and thus postcopy is inherantly out of scope. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|