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

[Andrea Righi <andrea.righi@canonical.com>]

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!
> >>