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

* 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.

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! >