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. Yep, sendfile() requires the input to be a mmapable FD, and the output to be a socket. Try splice() instead which merely requires 1 end to be a pipe, and the other end can be any FD afaik. 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 :|
