From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41278) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b2z3S-0006SO-LC for qemu-devel@nongnu.org; Wed, 18 May 2016 06:50:19 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1b2z3O-0008Sh-IK for qemu-devel@nongnu.org; Wed, 18 May 2016 06:50:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43164) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b2z3O-0008SV-D8 for qemu-devel@nongnu.org; Wed, 18 May 2016 06:50:14 -0400 Date: Wed, 18 May 2016 11:50:12 +0100 From: "Richard W.M. Jones" Message-ID: <20160518105011.GF1683@redhat.com> References: <20160516150401.GC28935@redhat.com> <20160516165336.GD15256@stefanha-x1.localdomain> <20160516182508.GQ1683@redhat.com> <573C1414.1050608@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <573C1414.1050608@linux.intel.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] Question about vNVDIMM file format List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Xiao Guangrong Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, "Zhang, Haozhong" On Wed, May 18, 2016 at 03:04:52PM +0800, Xiao Guangrong wrote: > On 05/17/2016 02:25 AM, Richard W.M. Jones wrote: > >(a) How necessary is the ACPI dependency? We disable ACPI because it > >is quite slow, adding something like 150-200ms to the boot process > >(every millisecond counts for us!). Because I previously never needed > >ACPI, I never really looked into why this is, and it could be > >something quite simple, so I'm going to look at this issue next. I > >understand that NVDIMMs are not regular (eg) PCI devices, so ordinary > >device probing isn't going to work, and that probably answers the > >question why you need to use ACPI. >=20 > Yes, ACPI is necessary to export NVDIMM devices. The good news is that > Intel is working on =E2=80=98lite QEMU=E2=80=99 which only has basic/si= mplest ACPI > support. Haozhong, who has been CCed, is working on it. I remeasured the ACPI overhead with the latest upstream kernel & qemu, it has dropped to under 20ms, so now I've just unconditionally enabled ACPI. > >(c) I've got the root filesystem (which is actually ext2, but using > >the ext4.ko driver) mounted with -o dax. What benefits / differences > >should I observe? Just general reduced memory / page cache usage? > > >=20 > And better performance as slow IO path is not needed anymore. :) >=20 > However, there is potential issue if it is not backend by real NVDIMM > hardware, the data is not persistent. We are going to resolve it by > emulating PCOMMIT and do msync properly. I'm using share=3Doff (ie. MMAP_PRIVATE), because for this appliance model I don't want writes to go to the backing disk. Rich. --=20 Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rj= ones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org