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! >>>