Re: [libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance
On Fri, Mar 18, 2022 at 02:34:29PM +0100, Claudio Fontana wrote:
...
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?
For what I remember (it was a long time ago sorry) I stopped to pursue
the POSIX_FADV_NOREUSE idea, because we thought that moving to a
memcg-based solution was a better and more flexible approach, assuming
memcg would have given some form of specific page cache control. As of
today I think we still don't have any specific page cache control
feature in memcg, so maybe we could reconsider the FADV_NOREUSE idea (or
something similar)?
Maybe even introduce a separate FADV_<something> flag if we don't want
to bind a specific implementation of this feature to a standard POSIX
flag (even if FADV_NOREUSE is still implemented as a no-op in the
kernel).
The thing that I liked about the fadvise approach is its simplicity from
an application perspective, because it's just a syscall and that's it,
without having to deal with any other subsystems (cgroups, sysfs, and
similar).
-Andrea
Lots of questions..
Thanks for all your insight,
Claudio
>
> 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!
>>