Re: [Ksummit-2010-discuss] checkpoint-restart: naked patch

On Sat, 6 Nov 2010, Matt Helsley wrote:

Yes, our patches touch a wide variety of kernel code. You have just failed
to appreciate how "wide" the kernel ABI truly is. You can't really count
it by number of syscalls, number of pseudo-filesystems, etc. There's
also the intended behavior of those interfaces to consider. Each piece
of checkpoint/restart code is relatively self-contained. This can be
confirmed merely by looking at many of the patches we've already posted
enabling checkpoint/restart of that feature. Until you've tried to
implement checkpoint/restart for an interface or until you've bothered
to review a patch for one of them (my favorite on is eventfd: ) please
don't tell us it's too complex. Then compare that with your proposed
ghastly stack of userspace cards -- ptrace (really more like strace) +
LD_PRELOAD + a daemon...

Incidentally, 20k lines of code is less than many pieces of the kernel.
It's less than many:

Filesystems (I've selected ones designed for rotating media or networks usually..)
ext4, nfs, ocfs2, xfs, reiserfs, ntfs, gfs2, jfs, cifs, ubifs, nilfs2, btrfs

Non-filesystem file-system support code:
nfsd, nls

It's less than one of the simpler DRM graphics drivers -- i915:
$ cd drivers/gpu/drm/i915
$ wc -l *.[ch]
41481 total

It's less than any one of the lpfc, bfa, aic7xxx, qla2xxx, and mpt2sas
drivers I see under scsi. Perhaps a more fair comparison might be to compare
a single driver to a single checkpointable kernel interface but it's
a more-fair comparison that skews even more in our favor.

Please, do not compare things like single file systems, drivers, or
otherwise fairly isolated components, with this "thing".
This thing touches a freaky-large number of subsystems, effectively
adding a glueage between them, which can might end up causing problems
(and/or restrict design choices) in the future.
The naked patch looks like just a sugar coating to me, which left out 300+
lines of extra logic in epoll alone.
This is one of the widest, deepest, intrusive patches I have seen in a
while, whose inclusion would require a little bit more than handwaving and
continuous re-posting IMO.

- Davide

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