Re: [Qemu-devel] Question about vNVDIMM file format
I thought you might be interested in some performance results, "hot off the presses". With DAX enabled, I see the following messages in the guest kernel logs, which I assume means it is working: [0.469364] EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk [0.469932] EXT4-fs (pmem0): mounting ext2 file system using the ext4 subsystem [0.470682] EXT4-fs (pmem0): mounted filesystem without journal. Opts: dax Enabling vNVDIMM + ext4.ko + DAX for the libguestfs appliance[1] root disk improves our boot+shutdown performance measure[2] by between 20 and 30 milliseconds, which is about 5% faster at the moment. I also wanted to know if memory usage is reduced. I ran `free -m' inside a freshly booted appliance. Without DAX: $ free -m totalusedfree shared buff/cache available Mem:485 3 451 1 30 465 Swap: 0 0 0 With DAX: $ free -m totalusedfree shared buff/cache available Mem:485 3 469 1 12 467 Swap: 0 0 0 I also wanted to know if qemu's memory usage is reduced. I captured the rusage.ru_maxrss of the qemu subprocess with and without DAX. The difference is only about 5 MB, which doesn't seem like very much to me. Perhaps I'm measuring this wrong. If you have better suggestions for measuring memory usage, please let me know. Rich. [1] http://libguestfs.org/guestfs-internals.1.html#architecture [2] http://libguestfs.org/guestfs-performance.1.html#baseline:-starting-the-appliance -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW
Re: [Qemu-devel] Question about vNVDIMM file format
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. > > Yes, ACPI is necessary to export NVDIMM devices. The good news is that > Intel is working on ‘lite QEMU’ which only has basic/simplest 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? > > > > And better performance as slow IO path is not needed anymore. :) > > 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=off (ie. MMAP_PRIVATE), because for this appliance model I don't want writes to go to the backing disk. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones 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
Re: [Qemu-devel] Question about vNVDIMM file format
Hi Richard, On 05/18/16 15:04, Xiao Guangrong wrote: [..] > >There are a few possible problems / questions I have: > > > >(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. > > Yes, ACPI is necessary to export NVDIMM devices. The good news is that > Intel is working on ‘lite QEMU’ which only has basic/simplest ACPI > support. Haozhong, who has been CCed, is working on it. > The way we were used for non-ACPI VM is really a dirty hack: we modified qemu and seabios to create a type-12 e820 entry for each pmem region, so that the guest linux could use the legacy pmem driver. According to ACPI 6 spec, type-7 e820 entries should be used here. However, the pmem driver in linux still requires ACPI for pmem present via type-7 e820 entries. (That is why we turned to type-12 and why I call it a hack.) We are now considering to add ACPI back in our project (for some other requirements than NVDIMM) and consequently will use the standard NVDIMM ACPI in QEMU, so above hack will be obsoleted in future. No optimization has been applied to NVDIMM ACPI so far. If the customized linux kernel is allowed, you could attempt to remove most ACPI supports and only leave NFIT (and maybe others that cannot be removed), which may save some boot time. [..] > > >(d) If, in future, you add the namespace metadata, what tools will be > >available on the host to create a packed filesystem + metadata? > >Assuming that we won't be able to export just a filesystem as I am > >doing now. > > Yes, this kind of tool is useful, we has this plan however it is low priority > in our TODO. :( > I'm not clear the exact requirement here. Maybe you could have a look at ndctl (https://github.com/pmem/ndctl) which supports some sorts of operations on namespace. Thanks, Haozhong
Re: [Qemu-devel] Question about vNVDIMM file format
Hi Rich, On 05/17/2016 02:25 AM, Richard W.M. Jones wrote: On Mon, May 16, 2016 at 09:53:36AM -0700, Stefan Hajnoczi wrote: On Mon, May 16, 2016 at 04:04:01PM +0100, Richard W.M. Jones wrote: I'm playing with ext4 and DAX. I'm using: -object memory-backend-file,id=mem1,share,mem-path=/var/tmp/pmem,size=4G \ -device nvdimm,memdev=mem1,id=nv1 where /var/tmp/pmem is a 4 GB ext4 filesystem image (no partition table). I can mount this in the guest using: mount -o dax /dev/pmem0 /mnt and everything appears to work. I read in the mailing list that the pmem file has some internal structure for storing config data, stored in the last 128 KB of the file. Is that still the case? AFAICT qemu.git/master does not support the ACPI _DSM for namespace configuration. That means the entire /var/tmp/pmem should be visible. That's great, thanks both for your answers. FWIW I was able to add support to libguestfs -- at least for the "direct" backend where we run qemu directly. Unfortunately libvirt does not support the vNVDIMM device yet. I have posted the two patches needed on our mailing list. There seems to be some delay in our mail server, so they aren't in the archives yet: https://www.redhat.com/archives/libguestfs/2016-May/thread.html There are a few possible problems / questions I have: (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. Yes, ACPI is necessary to export NVDIMM devices. The good news is that Intel is working on ‘lite QEMU’ which only has basic/simplest ACPI support. Haozhong, who has been CCed, is working on it. (b) Could you describe what the 3 modules (nd_btt, nd_pmem, nfit) do? Are all 3 modules necessary in the guest kernel? I think the best answer is from Kernel's Kconfig :): ACPI_NFIT: Infrastructure to probe ACPI 6 compliant platforms for NVDIMMs (NFIT) and register a libnvdimm device tree BTT: The Block Translation Table (BTT) provides atomic sector update semantics for persistent memory devices, so that applications that rely on sector writes not being torn (a guarantee that typical disks provide) can continue to do so. PMEM: Memory ranges for PMEM are described by either an NFIT (NVDIMM Firmware Interface Table, see CONFIG_NFIT_ACPI), a non-standard OEM-specific E820 memory type (type-12, see CONFIG_X86_PMEM_LEGACY), or it is manually specified by the 'memmap=nn[KMG]!ss[KMG]' kernel command line (see Documentation/kernel-parameters.txt). This driver converts these persistent memory ranges into block devices that are capable of DAX (direct-access) file system mappings Currently vNVDIMM is pure PMEM device without label, BTT is unnecessary, so you can say N to BTT when configuring linux kernel for VM. (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? And better performance as slow IO path is not needed anymore. :) 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. (d) If, in future, you add the namespace metadata, what tools will be available on the host to create a packed filesystem + metadata? Assuming that we won't be able to export just a filesystem as I am doing now. Yes, this kind of tool is useful, we has this plan however it is low priority in our TODO. :(
Re: [Qemu-devel] Question about vNVDIMM file format
On Mon, May 16, 2016 at 04:04:01PM +0100, Richard W.M. Jones wrote: > I'm playing with ext4 and DAX. > > I'm using: > > -object memory-backend-file,id=mem1,share,mem-path=/var/tmp/pmem,size=4G \ > -device nvdimm,memdev=mem1,id=nv1 > > where /var/tmp/pmem is a 4 GB ext4 filesystem image (no partition > table). I can mount this in the guest using: > > mount -o dax /dev/pmem0 /mnt > > and everything appears to work. > > I read in the mailing list that the pmem file has some internal > structure for storing config data, stored in the last 128 KB of the > file. Is that still the case? AFAICT qemu.git/master does not support the ACPI _DSM for namespace configuration. That means the entire /var/tmp/pmem should be visible. Stefan signature.asc Description: PGP signature
Re: [Qemu-devel] Question about vNVDIMM file format
On Mon, May 16, 2016 at 09:53:36AM -0700, Stefan Hajnoczi wrote: > On Mon, May 16, 2016 at 04:04:01PM +0100, Richard W.M. Jones wrote: > > I'm playing with ext4 and DAX. > > > > I'm using: > > > > -object memory-backend-file,id=mem1,share,mem-path=/var/tmp/pmem,size=4G \ > > -device nvdimm,memdev=mem1,id=nv1 > > > > where /var/tmp/pmem is a 4 GB ext4 filesystem image (no partition > > table). I can mount this in the guest using: > > > > mount -o dax /dev/pmem0 /mnt > > > > and everything appears to work. > > > > I read in the mailing list that the pmem file has some internal > > structure for storing config data, stored in the last 128 KB of the > > file. Is that still the case? > > AFAICT qemu.git/master does not support the ACPI _DSM for namespace > configuration. That means the entire /var/tmp/pmem should be visible. That's great, thanks both for your answers. FWIW I was able to add support to libguestfs -- at least for the "direct" backend where we run qemu directly. Unfortunately libvirt does not support the vNVDIMM device yet. I have posted the two patches needed on our mailing list. There seems to be some delay in our mail server, so they aren't in the archives yet: https://www.redhat.com/archives/libguestfs/2016-May/thread.html There are a few possible problems / questions I have: (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. (b) Could you describe what the 3 modules (nd_btt, nd_pmem, nfit) do? Are all 3 modules necessary in the guest kernel? (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? (d) If, in future, you add the namespace metadata, what tools will be available on the host to create a packed filesystem + metadata? Assuming that we won't be able to export just a filesystem as I am doing now. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html
Re: [Qemu-devel] Question about vNVDIMM file format
On 05/16/2016 11:04 PM, Richard W.M. Jones wrote: I'm playing with ext4 and DAX. Thanks for your try, Rich! :) I'm using: -object memory-backend-file,id=mem1,share,mem-path=/var/tmp/pmem,size=4G \ -device nvdimm,memdev=mem1,id=nv1 where /var/tmp/pmem is a 4 GB ext4 filesystem image (no partition table). I can mount this in the guest using: mount -o dax /dev/pmem0 /mnt and everything appears to work. I read in the mailing list that the pmem file has some internal structure for storing config data, stored in the last 128 KB of the file. Is that still the case? The patchset supporting label data has not been merged yet so currently there is no label data for vNVDIMM device.
