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

[Claudio Fontana]

On 3/25/22 12:14 PM, Daniel P. Berrangé wrote:
> On Fri, Mar 25, 2022 at 11:56:44AM +0100, Claudio Fontana wrote:
>> Thanks Daniel,
>>
>> On 3/25/22 11:33 AM, 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.
>>>>
>>>> One thing that I noticed is:
>>>>
>>>> ommit afe6e58aedcd5e27ea16184fed90b338569bd042
>>>> Author: Jiri Denemark <jdenemar@redhat.com>
>>>> Date:   Mon Feb 6 14:40:48 2012 +0100
>>>>
>>>>     util: Generalize virFileDirectFd
>>>>     
>>>>     virFileDirectFd was used for accessing files opened with O_DIRECT using
>>>>     libvirt_iohelper. We will want to use the helper for accessing files
>>>>     regardless on O_DIRECT and thus virFileDirectFd was generalized and
>>>>     renamed to virFileWrapperFd.
>>>>
>>>>
>>>> And in particular the comment in src/util/virFile.c:
>>>>
>>>>     /* XXX support posix_fadvise rather than O_DIRECT, if the kernel 
>>>> support                                                                    
>>>>                              
>>>>      * for that is decent enough. In that case, we will also need to       
>>>>                                                                            
>>>>                        
>>>>      * explicitly support VIR_FILE_WRAPPER_NON_BLOCKING since              
>>>>                                                                            
>>>>                        
>>>>      * VIR_FILE_WRAPPER_BYPASS_CACHE alone will no longer require spawning 
>>>>                                                                            
>>>>                        
>>>>      * iohelper.                                                           
>>>>                                                                            
>>>>                        
>>>>      */
>>>>
>>>> by Jiri Denemark.
>>>>
>>>> I have lots of questions here, and I tried to involve Jiri and Andrea 
>>>> Righi here, who a long time ago proposed a POSIX_FADV_NOREUSE 
>>>> implementation.
>>>>
>>>> 1) What is the reason iohelper was introduced?
>>>
>>> With POSIX you can't get sensible results from poll() on FDs associated with
>>> plain files. It will always report the file as readable/writable, and the
>>> userspace caller will get blocked any time the I/O operation causes the
>>> kernel to read/write from the underlying (potentially very slow) storage.
>>>
>>> IOW if you give QEMU an FD associated with a plain file and tell it to
>>> migrate to that, the guest OS will get stalled.
>>
>> we send a stop command to qemu just before migrating to a file in virsh save 
>> though right?
>> With virsh restore we also first load the VM, and only then start executing 
>> it.
>>
>> So for virsh save and virsh restore, this should not be a problem? Still we 
>> need the iohelper?
> 
> The same code is used in libvirt for other commands like 'virsh dump'
> and snapshots, where the VM remains live though. In general I don't
> think we should remove the iohelper, because QEMU code is written from
> the POV that the channels honour O_NOBLOCK.
> 

understand.. it is actually not traceful to QEMU anyway indeed. Thanks for the 
clarification.

Claudio