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

[Claudio Fontana]

On 4/7/22 3:53 PM, Dr. David Alan Gilbert wrote:
> * Claudio Fontana (cfontana@suse.de) wrote:
>> On 4/5/22 10:35 AM, Dr. David Alan Gilbert wrote:
>>> * Claudio Fontana (cfontana@suse.de) wrote:
>>>> 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
>>>>>
>>>>
>>>> Crazy idea, would trying to use the parallel migration concept for 
>>>> migrating to/from a file make any sense?
>>>>
>>>> Not sure if applying the qemu multifd implementation of this would apply, 
>>>> maybe it could be given another implementation for "toFile", trying to use 
>>>> more than one cpu to do the transfer?
>>>
>>> I can't see a way that would help; well, I could if you could
>>> somehow have multiple io helper threads that dealt with it.
>>
>> The first issue I encounter here for both the "virsh save" and "virsh 
>> restore" scenarios is that libvirt uses fd: migration, not unix: migration.
>> QEMU supports multifd for unix:, tcp:, vsock: as far as I can see.
>>
>> Current save procedure in QMP in short:
>>
>> {"execute":"migrate-set-capabilities", ...}
>> {"execute":"migrate-set-parameters", ...}
>> {"execute":"getfd","arguments":{"fdname":"migrate"}, ...} fd=26
>> QEMU_MONITOR_IO_SEND_FD: fd=26
>> {"execute":"migrate","arguments":{"uri":"fd:migrate"}, ...}
>>
>>
>> Current restore procedure in QMP in short:
>>
>> (start QEMU)
>> {"execute":"migrate-incoming","arguments":{"uri":"fd:21"}, ...}
>>
>>
>> Should I investigate changing libvirt to use unix: for save/restore?
>> Or should I look into changing qemu to somehow accept fd: for multifd, 
>> meaning I guess providing multiple fd: uris in the migrate command?
> 
> So I'm not sure this is the right direction; i.e. if multifd is the
> right answer to your problem.

Of course, just exploring the space.

> However, I think the qemu code probably really really wants to be a
> socket.

Understood, I'll try to bend libvirt to use unix:/// and see how far I get,

Thanks,

Claudio

> 
> Dave
> 
>>
>> Thank you for your help,
>>
>> Claudio
>>