From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47755) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1efsPc-0003GK-8V for qemu-devel@nongnu.org; Sun, 28 Jan 2018 14:14:45 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1efsPY-0002Ys-Vq for qemu-devel@nongnu.org; Sun, 28 Jan 2018 14:14:44 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51262) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1efsPY-0002XQ-Pc for qemu-devel@nongnu.org; Sun, 28 Jan 2018 14:14:40 -0500 From: Juan Quintela In-Reply-To: <20180126155940.26994-1-danielhb@linux.vnet.ibm.com> (Daniel Henrique Barboza's message of "Fri, 26 Jan 2018 13:59:40 -0200") References: <20180126155940.26994-1-danielhb@linux.vnet.ibm.com> Reply-To: quintela@redhat.com Date: Sun, 28 Jan 2018 20:14:41 +0100 Message-ID: <87shapre1q.fsf@secure.laptop> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [PATCH v2 1/1] migration/savevm.c: set MAX_VM_CMD_PACKAGED_SIZE to 1ul << 32 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Daniel Henrique Barboza Cc: qemu-devel@nongnu.org, dgilbert@redhat.com Daniel Henrique Barboza wrote: > MAX_VM_CMD_PACKAGED_SIZE is a constant used in qemu_savevm_send_packaged > and loadvm_handle_cmd_packaged to determine whether a package is too > big to be sent or received. qemu_savevm_send_packaged is called inside > postcopy_start (migration/migration.c) to send the MigrationState > in a single blob to the destination, using the MIG_CMD_PACKAGED subcommand, > which will read it up using loadvm_handle_cmd_packaged. If the blob is > larger than MAX_VM_CMD_PACKAGED_SIZE, an error is thrown and the postcopy > migration is aborted. Both MAX_VM_CMD_PACKAGED_SIZE and MIG_CMD_PACKAGED > were introduced by commit 11cf1d984b ("MIG_CMD_PACKAGED: Send a packaged > chunk ..."). The constant has its original value of 1ul << 24 (16MB). > > The current MAX_VM_CMD_PACKAGED_SIZE value is not enough to support postcopy > migration of bigger pseries guests. The blob size for a postcopy migration of > a pseries guest with the following setup: > > qemu-system-ppc64 --nographic -vga none -machine pseries,accel=kvm -m 64G \ > -smp 1,maxcpus=32 -device virtio-blk-pci,drive=rootdisk \ > -drive file=f27.qcow2,if=none,cache=none,format=qcow2,id=rootdisk \ > -netdev user,id=u1 -net nic,netdev=u1 > > Goes around 12MB. Bumping the RAM to 128G makes the blob sizes goes to 20MB. > With 256G the blob goes to 37MB - more than twice the current maximum size. > At this moment the pseries machine can handle guests with up to 1TB of RAM, > making this postcopy blob goes to 128MB of size approximately. > > Following the discussions made in [1], there is a need to understand what > devices are aggressively consuming the blob in that manner and see if that > can be mitigated. Until then, we can set MAX_VM_CMD_PACKAGED_SIZE to the > maximum value allowed. Since the size is a 32 bit int variable, we can set > it as 1ul << 32, giving a maximum blob size of 4G that is enough to support > postcopy migration of 32TB RAM guests given the above constraints. > > [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg06313.html > > Signed-off-by: Daniel Henrique Barboza > Reported-by: Balamuruhan S Reviewed-by: Juan Quintela