* 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. Dave > Ciao, > > C > > >> > >> In the above tests with libvirt, were you using the > >> --bypass-cache flag or not ? > > > > No, I do not. Tests with ramdisk did not show a notable difference for me, > > > > but tests with /dev/null were not possible, since the command line is not accepted: > > > > # virsh save centos7 /dev/null > > Domain 'centos7' saved to /dev/null > > [OK] > > > > # virsh save centos7 /dev/null --bypass-cache > > error: Failed to save domain 'centos7' to /dev/null > > error: Failed to create file '/dev/null': Invalid argument > > > > > >> > >> Hopefully use of O_DIRECT doesn't make a difference for > >> /dev/null, since the I/O is being immediately thrown > >> away and so ought to never go into I/O cache. > >> > >> In terms of the comparison, we still have libvirt iohelper > >> giving QEMU a pipe, while your test above gives QEMU a > >> UNIX socket. > >> > >> So I still wonder if the delta is caused by the pipe vs socket > >> difference, as opposed to netcat vs libvirt iohelper code. > > > > I'll look into this aspect, thanks! > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
