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? 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? Lots of questions.. Thanks for all your insight, Claudio
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!