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

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).

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!