[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

[Dr. David Alan Gilbert]

* Claudio Fontana (cfontana at 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 at 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.

Dave

> 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!
> 
-- 
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK