On 3/25/22 12:14 PM, Daniel P. Berrangé wrote: > 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@xxxxxxx) 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@xxxxxxx> >>>>>>>>>>>>>> --- >>>>>>>>>>>>>> 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@xxxxxxxxxx> >>>> 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. > understand.. it is actually not traceful to QEMU anyway indeed. Thanks for the clarification. Claudio
