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

On 3/25/22 12:29 PM, 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(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. > > Yep, sendfile() requires the input to be a mmapable FD, > and the output to be a socket. > > Try splice() instead which merely requires 1 end to be a > pipe, and the other end can be any FD afaik. > > With regards, > Daniel > I did try splice(), but performance is worse by around 500%. It also fails with EINVAL when trying to use it in combination with O_DIRECT. Tried larger and smaller buffers, flags like SPLICE_F_MORE an SPLICE_F_MOVE in any combination; no change, just awful performance.

Here is the code: #ifdef __linux__ +static ssize_t safesplice(int fdin, int fdout, size_t todo) +{ + unsigned int flags = SPLICE_F_MOVE | SPLICE_F_MORE; + ssize_t ncopied = 0; + + while (todo > 0) { + ssize_t r = splice(fdin, NULL, fdout, NULL, todo, flags); + if (r < 0 && errno == EINTR) + continue; + if (r < 0) + return r; + if (r == 0) + return ncopied; + todo -= r; + ncopied += r; + } + return ncopied; +} + +static ssize_t runIOCopy(const struct runIOParams p) +{ + size_t len = 1024 * 1024; + ssize_t total = 0; + + while (1) { + ssize_t got = safesplice(p.fdin, p.fdout, len); + if (got < 0) + return -1; + if (got == 0) + break; + + total += got; + + /* handle last write truncate in direct case */ + if (got < len && p.isDirect && p.isWrite && !p.isBlockDev) { + if (ftruncate(p.fdout, total) < 0) { + return -4; + } + break; + } + } + return total; +} + +#endif Any ideas welcome, Claudio