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

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 ?

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.

With regards, Daniel