Re: [libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

On Fri, Mar 25, 2022 at 11:56:44AM +0100, Claudio Fontana wrote: > Thanks Daniel, > > On 3/25/22 11:33 AM, Daniel P. Berrangé wrote: >> On Fri, Mar 18, 2022 at 02:34:29PM +0100, Claudio Fontana wrote: >>> On 3/17/22 4:03 PM, Dr. David Alan Gilbert wrote: >>>> * Claudio Fontana (cfontana(a)suse.de) wrote: >>>>> On 3/17/22 2:41 PM, Claudio Fontana wrote: >>>>>> On 3/17/22 11:25 AM, Daniel P. Berrangé wrote: >>>>>>> On Thu, Mar 17, 2022 at 11:12:11AM +0100, Claudio Fontana wrote: >>>>>>>> On 3/16/22 1:17 PM, Claudio Fontana wrote: >>>>>>>>> On 3/14/22 6:48 PM, Daniel P. Berrangé wrote: >>>>>>>>>> On Mon, Mar 14, 2022 at 06:38:31PM +0100, Claudio Fontana wrote: >>>>>>>>>>> On 3/14/22 6:17 PM, Daniel P. Berrangé wrote: >>>>>>>>>>>> On Sat, Mar 12, 2022 at 05:30:01PM +0100, Claudio Fontana wrote: >>>>>>>>>>>>> the first user is the qemu driver, >>>>>>>>>>>>> >>>>>>>>>>>>> virsh save/resume would slow to a crawl with a default pipe size (64k). >>>>>>>>>>>>> >>>>>>>>>>>>> This improves the situation by 400%. >>>>>>>>>>>>> >>>>>>>>>>>>> Going through io_helper still seems to incur in some penalty (~15%-ish) >>>>>>>>>>>>> compared with direct qemu migration to a nc socket to a file. >>>>>>>>>>>>> >>>>>>>>>>>>> Signed-off-by: Claudio Fontana <cfontana(a)suse.de&gt; >>>>>>>>>>>>> --- >>>>>>>>>>>>> src/qemu/qemu_driver.c | 6 +++--- >>>>>>>>>>>>> src/qemu/qemu_saveimage.c | 11 ++++++----- >>>>>>>>>>>>> src/util/virfile.c | 12 ++++++++++++ >>>>>>>>>>>>> src/util/virfile.h | 1 + >>>>>>>>>>>>> 4 files changed, 22 insertions(+), 8 deletions(-) >>>>>>>>>>>>> >>>>>>>>>>>>> Hello, I initially thought this to be a qemu performance issue, >>>>>>>>>>>>> so you can find the discussion about this in qemu-devel: >>>>>>>>>>>>> >>>>>>>>>>>>> "Re: bad virsh save /dev/null performance (600 MiB/s max)" >>>>>>>>>>>>> >>>>>>>>>>>>> https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg03142.html >>>>>>> >>>>>>> >>>>>>>> Current results show these experimental averages maximum throughput >>>>>>>> migrating to /dev/null per each FdWrapper Pipe Size (as per QEMU QMP >>>>>>>> "query-migrate", tests repeated 5 times for each). >>>>>>>> VM Size is 60G, most of the memory effectively touched before migration, >>>>>>>> through user application allocating and touching all memory with >>>>>>>> pseudorandom data. >>>>>>>> >>>>>>>> 64K: 5200 Mbps (current situation) >>>>>>>> 128K: 5800 Mbps >>>>>>>> 256K: 20900 Mbps >>>>>>>> 512K: 21600 Mbps >>>>>>>> 1M: 22800 Mbps >>>>>>>> 2M: 22800 Mbps >>>>>>>> 4M: 22400 Mbps >>>>>>>> 8M: 22500 Mbps >>>>>>>> 16M: 22800 Mbps >>>>>>>> 32M: 22900 Mbps >>>>>>>> 64M: 22900 Mbps >>>>>>>> 128M: 22800 Mbps >>>>>>>> >>>>>>>> This above is the throughput out of patched libvirt with multiple Pipe Sizes for the FDWrapper. >>>>>>> >>>>>>> Ok, its bouncing around with noise after 1 MB. So I'd suggest that >>>>>>> libvirt attempt to raise the pipe limit to 1 MB by default, but >>>>>>> not try to go higher. >>>>>>> >>>>>>>> As for the theoretical limit for the libvirt architecture, >>>>>>>> I ran a qemu migration directly issuing the appropriate QMP >>>>>>>> commands, setting the same migration parameters as per libvirt, >>>>>>>> and then migrating to a socket netcatted to /dev/null via >>>>>>>> {"execute": "migrate", "arguments": { "uri", "unix:///tmp/netcat.sock" } } : >>>>>>>> >>>>>>>> QMP: 37000 Mbps >>>>>>> >>>>>>>> So although the Pipe size improves things (in particular the >>>>>>>> large jump is for the 256K size, although 1M seems a very good value), >>>>>>>> there is still a second bottleneck in there somewhere that >>>>>>>> accounts for a loss of ~14200 Mbps in throughput. >>>>> >>>>> >>>>> Interesting addition: I tested quickly on a system with faster cpus and larger VM sizes, up to 200GB, >>>>> and the difference in throughput libvirt vs qemu is basically the same ~14500 Mbps. >>>>> >>>>> ~50000 mbps qemu to netcat socket to /dev/null >>>>> ~35500 mbps virsh save to /dev/null >>>>> >>>>> Seems it is not proportional to cpu speed by the looks of it (not a totally fair comparison because the VM sizes are different). >>>> >>>> It might be closer to RAM or cache bandwidth limited though; for an extra copy. >>> >>> I was thinking about sendfile(2) in iohelper, but that probably can't work as the input fd is a socket, I am getting EINVAL. >>> >>> One thing that I noticed is: >>> >>> ommit afe6e58aedcd5e27ea16184fed90b338569bd042 >>> Author: Jiri Denemark <jdenemar(a)redhat.com&gt; >>> Date: Mon Feb 6 14:40:48 2012 +0100 >>> >>> util: Generalize virFileDirectFd >>> >>> virFileDirectFd was used for accessing files opened with O_DIRECT using >>> libvirt_iohelper. We will want to use the helper for accessing files >>> regardless on O_DIRECT and thus virFileDirectFd was generalized and >>> renamed to virFileWrapperFd. >>> >>> >>> And in particular the comment in src/util/virFile.c: >>> >>> /* XXX support posix_fadvise rather than O_DIRECT, if the kernel support >>> * for that is decent enough. In that case, we will also need to >>> * explicitly support VIR_FILE_WRAPPER_NON_BLOCKING since >>> * VIR_FILE_WRAPPER_BYPASS_CACHE alone will no longer require spawning >>> * iohelper. >>> */ >>> >>> by Jiri Denemark. >>> >>> I have lots of questions here, and I tried to involve Jiri and Andrea Righi here, who a long time ago proposed a POSIX_FADV_NOREUSE implementation. >>> >>> 1) What is the reason iohelper was introduced? >> >> With POSIX you can't get sensible results from poll() on FDs associated with >> plain files. It will always report the file as readable/writable, and the >> userspace caller will get blocked any time the I/O operation causes the >> kernel to read/write from the underlying (potentially very slow) storage. >> >> IOW if you give QEMU an FD associated with a plain file and tell it to >> migrate to that, the guest OS will get stalled. > > we send a stop command to qemu just before migrating to a file in virsh save though right? > With virsh restore we also first load the VM, and only then start executing it. > > So for virsh save and virsh restore, this should not be a problem? Still we need the iohelper? The same code is used in libvirt for other commands like 'virsh dump' and snapshots, where the VM remains live though. In general I don't think we should remove the iohelper, because QEMU code is written from the POV that the channels honour O_NOBLOCK.