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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Mar 18, 2022 at 02:34:29PM +0100, Claudio Fontana wrote:
> On 3/17/22 4:03 PM, Dr. David Alan Gilbert wrote:
> > * Claudio Fontana (cfontana@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@suse.de>
> >>>>>>>>>> ---
> >>>>>>>>>>  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.
> 
> I was thinking about sendfile(2) in iohelper, but that probably
> can't work as the input fd is a socket, I am getting EINVAL.

Yep, sendfile() requires the input to be a mmapable FD,
and the output to be a socket.

Try splice() instead  which merely requires 1 end to be a
pipe, and the other end can be any FD afaik.

With regards,
Daniel
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