Hi 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. > > To avoid this we have to give QEMU an FD that is NOT a plain file, but > rather something on which poll() works correctly to avoid blocking. This > essentially means a socket or pipe FD. > > Here enters the iohelper - we give QEMU a pipe whose other end is the > iohelper. The iohelper suffers from blocking on read/write but that > doesn't matter, because QEMU is isolated from this via the pipe. I am still puzzled by this, when we migrate to a file via virsh save in qemu_saveimage.c , we suspend the guest anyway right? But maybe there is some other problem that triggers? In the Restore code, ie qemuSaveImageOpen(), we say: if (bypass_cache && !(*wrapperFd = virFileWrapperFdNew(&fd, path, VIR_FILE_WRAPPER_BYPASS_CACHE))) return -1; why don't we make the wrapper conditional on bypass_cache in the Save code too, in qemuSaveImageCreate? I ask because I tried this change: commit ae7dff45f10be78d1555e3f302f337e72afa300c Author: Claudio Fontana <cfontana@xxxxxxx> Date: Sun Apr 10 12:33:37 2022 -0600 only use wrapper if you want to skip the filesystem cache Signed-off-by: Claudio Fontana <cfontana@xxxxxxx> diff --git a/src/qemu/qemu_saveimage.c b/src/qemu/qemu_saveimage.c index 4fd4c5cfcd..5ea1b2fbcc 100644 --- a/src/qemu/qemu_saveimage.c +++ b/src/qemu/qemu_saveimage.c @@ -289,8 +289,10 @@ qemuSaveImageCreate(virQEMUDriver *driver, if (qemuSecuritySetImageFDLabel(driver->securityManager, vm->def, fd) < 0) goto cleanup; - if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) - goto cleanup; + if ((flags & VIR_DOMAIN_SAVE_BYPASS_CACHE)) { + if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) + goto cleanup; + } if (virQEMUSaveDataWrite(data, fd, path) < 0) goto cleanup; and I got a pretty good performance improvement, where it would be better in my use case not to use O_DIRECT anymore, and nothing prohibits to still use O_DIRECT if desired. I get these results with a 90 G VM with this patch applied: # echo 3 > /proc/sys/vm/drop_caches # time virsh save centos7 /vm_images/claudio/savevm --bypass-cache Domain 'centos7' saved to /vm_images/claudio/savevm real 2m9.368s # echo 3 > /proc/sys/vm/drop_caches # time virsh save centos7 /vm_images/claudio/savevm Domain 'centos7' saved to /vm_images/claudio/savevm real 0m42.155s and now without this patch applied: # echo 3 > /proc/sys/vm/drop_caches # time virsh save centos7 /vm_images/claudio/savevm --bypass-cache Domain 'centos7' saved to /vm_images/claudio/savevm real 2m10.468s # echo 3 > /proc/sys/vm/drop_caches # time virsh save centos7 /vm_images/claudio/savevm Domain 'centos7' saved to /vm_images/claudio/savevm real 2m6.142s I'll rerun the numbers again next week on a machine with better cpu if possible. Thanks, Claudio > > In theory we could just spawn a thread inside libvirtd todo the same > as the iohelper, but using a separate helper process is more robust > > If not using libvirt, you would use QEMU's 'exec:' migration protocol > with 'dd' or 'cat' for the same reasons. Libvirt provides the iohelper > so we don't have to deal with portibility questions around 'dd' syntax > and can add features like O_DIRECT that cat lacks. > >> 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? > > We can't remove the iohelper for the reason above. > >> 4) What has stopped Andreas' or another POSIX_FADV_NOREUSE implementation in the kernel? > > With regards, > Daniel >
