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

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

On Fri, Mar 25, 2022 at 11:56:44AM +0100, Claudio Fontana wrote:
> Thanks Daniel,
> 
> On 3/25/22 11:33 AM, Daniel P. Berrangé wrote:
> > 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.
> >>
> >> One thing that I noticed is:
> >>
> >> ommit afe6e58aedcd5e27ea16184fed90b338569bd042
> >> Author: Jiri Denemark <jdenemar@redhat.com>
> >> Date:   Mon Feb 6 14:40:48 2012 +0100
> >>
> >>     util: Generalize virFileDirectFd
> >>     
> >>     virFileDirectFd was used for accessing files opened with O_DIRECT using
> >>     libvirt_iohelper. We will want to use the helper for accessing files
> >>     regardless on O_DIRECT and thus virFileDirectFd was generalized and
> >>     renamed to virFileWrapperFd.
> >>
> >>
> >> And in particular the comment in src/util/virFile.c:
> >>
> >>     /* XXX support posix_fadvise rather than O_DIRECT, if the kernel support                                                                                                 
> >>      * for that is decent enough. In that case, we will also need to                                                                                                         
> >>      * explicitly support VIR_FILE_WRAPPER_NON_BLOCKING since                                                                                                                
> >>      * VIR_FILE_WRAPPER_BYPASS_CACHE alone will no longer require spawning                                                                                                   
> >>      * iohelper.                                                                                                                                                             
> >>      */
> >>
> >> by Jiri Denemark.
> >>
> >> I have lots of questions here, and I tried to involve Jiri and Andrea Righi here, who a long time ago proposed a POSIX_FADV_NOREUSE implementation.
> >>
> >> 1) What is the reason iohelper was introduced?
> > 
> > With POSIX you can't get sensible results from poll() on FDs associated with
> > plain files. It will always report the file as readable/writable, and the
> > userspace caller will get blocked any time the I/O operation causes the
> > kernel to read/write from the underlying (potentially very slow) storage.
> > 
> > IOW if you give QEMU an FD associated with a plain file and tell it to
> > migrate to that, the guest OS will get stalled.
> 
> we send a stop command to qemu just before migrating to a file in virsh save though right?
> With virsh restore we also first load the VM, and only then start executing it.
> 
> So for virsh save and virsh restore, this should not be a problem? Still we need the iohelper?

The same code is used in libvirt for other commands like 'virsh dump'
and snapshots, where the VM remains live though. In general I don't
think we should remove the iohelper, because QEMU code is written from
the POV that the channels honour O_NOBLOCK.

With regards,
Daniel
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