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@xxxxxxx) 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@xxxxxxx> > >>>>>>>>>>>> --- > >>>>>>>>>>>> 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@xxxxxxxxxx> > >> 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. 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 :|
