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!