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

On 3/17/22 4:03 PM, Dr. David Alan Gilbert wrote: > * Claudio Fontana (cfontana(a)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(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. >>>> >>>> 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(a)redhat.com&gt; 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?

2) Was Jiri's comment about the missing linux implementation of POSIX_FADV_NOREUSE? 3) if using O_DIRECT is the only reason for iohelper to exist (...?), would replacing it with posix_fadvise remove the need for iohelper?

4) What has stopped Andreas' or another POSIX_FADV_NOREUSE implementation in the kernel?