On 3/28/22 12:47 PM, Claudio Fontana wrote:
On 3/26/22 4:49 PM, Claudio Fontana wrote:
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@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.
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.
Ok I found a case where splice actually helps: in the read case, without O_DIRECT, splice seems to actually outperform read/write by _a lot_.
I was just hit by a cache effect. No real improvements I could measure.
I will code up the patch and start making more experiments with larger VM sizes etc.
Thanks!
Claudio
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