[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

[Claudio Fontana]

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 at 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.
> 
> In the above tests with libvirt, were you using the
> --bypass-cache flag or not ?

No, I do not. Tests with ramdisk did not show a notable difference for me,

but tests with /dev/null were not possible, since the command line is not accepted:

# virsh save centos7 /dev/null
Domain 'centos7' saved to /dev/null
[OK]

# virsh save centos7 /dev/null --bypass-cache
error: Failed to save domain 'centos7' to /dev/null
error: Failed to create file '/dev/null': Invalid argument


> 
> Hopefully use of O_DIRECT doesn't make a difference for
> /dev/null, since the I/O is being immediately thrown
> away and so ought to never go into I/O cache. 
> 
> In terms of the comparison, we still have libvirt iohelper
> giving QEMU a pipe, while your test above gives QEMU a
> UNIX socket.
> 
> So I still wonder if the delta is caused by the pipe vs socket
> difference, as opposed to netcat vs libvirt iohelper code.

I'll look into this aspect, thanks!
> 
> With regards,
> Daniel
> 

Ciao,

Claudio