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

[Claudio Fontana <cfontana@suse.de>]

On 3/28/22 10:31 AM, Daniel P. Berrangé wrote:
> On Sat, Mar 26, 2022 at 04:49:46PM +0100, 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.
>>>
>>
>> I did try splice(), but performance is worse by around 500%.
> 
> Hmm, that's certainly unexpected !
> 
>> Any ideas welcome,
> 
> I learnt there is also a newer  copy_file_range call, not sure if that's
> any better.
> 
> You passed len as 1 MB, I wonder if passing MAXINT is viable ? We just
> want to copy everything IIRC.
> 
> With regards,
> Daniel
> 

Hi Daniel, tried also up to 64MB, no improvement with splice.

I'll take a look at copy_file_range,

Thanks!

Claudio