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

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Date:	Sun, 7 Nov 2010 14:59:27 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Matt Helsley <matthltc@...ibm.com>
cc:	Tejun Heo <tj@...nel.org>, Oren Laadan <orenl@...columbia.edu>,
	ksummit-2010-discuss@...ts.linux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: 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:
> http://www.mail-archive.com/devel@openvz.org/msg21565.html ) 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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