* Claudio Fontana (cfont...@suse.de) wrote: > On 3/28/22 10:31 AM, Daniel P. Berrangé wrote: > > On Sat, Mar 26, 2022 at 04:49:46PM +0100, Claudio Fontana wrote: > >> On 3/25/22 12:29 PM, 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 (cfont...@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 <cfont...@suse.de> > >>>>>>>>>>>>>> --- > >>>>>>>>>>>>>> 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. > >>> > >> > >> I did try splice(), but performance is worse by around 500%. > > > > Hmm, that's certainly unexpected ! > > > >> Any ideas welcome, > > > > I learnt there is also a newer copy_file_range call, not sure if that's > > any better. > > > > You passed len as 1 MB, I wonder if passing MAXINT is viable ? We just > > want to copy everything IIRC. > > > > With regards, > > Daniel > > > > Crazy idea, would trying to use the parallel migration concept for migrating > to/from a file make any sense? > > Not sure if applying the qemu multifd implementation of this would apply, > maybe it could be given another implementation for "toFile", trying to use > more than one cpu to do the transfer?
I can't see a way that would help; well, I could if you could somehow have multiple io helper threads that dealt with it. Dave > Thanks, > > Claudio > > > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
