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

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.

> > 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!