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

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.

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.