[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

Claudio Fontana cfontana at suse.de
Fri Mar 18 13:34:29 UTC 2022

On 3/17/22 4:03 PM, Dr. David Alan Gilbert wrote:
> * Claudio Fontana (cfontana at 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 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.
>> 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 at 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?

2) Was Jiri's comment about the missing linux implementation of POSIX_FADV_NOREUSE?

3) if using O_DIRECT is the only reason for iohelper to exist (...?), would replacing it with posix_fadvise remove the need for iohelper?

4) What has stopped Andreas' or another POSIX_FADV_NOREUSE implementation in the kernel?

Lots of questions..

Thanks for all your insight,


> Dave
>> Ciao,
>> C
>>>> 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!

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