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SubjectRE: Ping? RE: [GIT PULL] mm/vfs/fs:cleancache for 2.6.37 merge window
> From: Andrew Morton [mailto:akpm@linux-foundation.org]
> So can we please revisit all this from the top level?
> Jeremy, your input would be valuable.

Hi Andrew --

Thanks for your reply! Between preparing for LPC and some
upcoming personal time off, I may not be able to reply in
a timely way to some future discussion on this thread, so
I will try to respond now but still encourage others to
respond. I would also be happy to talk f2f at LPC. I see as
I type this that Jeremy has already replied and will try
to incorporate information re his comments.

Andrew, I think you raise four interesting questions...
I hope it's OK if I paraphrase rather than quote directly?

1) Is something Xen-specific [though no, it's not] worth
the cost of this addition to the code base?
2) Even if (1) is yes, is this going to be used by a
significant percentage of Xen users?
3) Is cleancache beneficial to NON-Xen users?
4) Ignoring the "user-base" questions, are there technical
objections to and issues with cleancache that would
stop it from being merged?
So, I hope you have the time to read my long-winded reply:

1) By using the term, "micro-audience" to refer to the xen
user base, I think you are grossly minimizing their
number. Since this is a technical list, we can leave
it up to industry analysts to argue that point, but I
think it is at least fair to point out that there are
literally dozens of merges accepted in just this window
that had a much larger code impact than cleancache
but have a much smaller user base than Xen.

While it is reasonable to argue that most of those other
merges don't touch core mm/vfs code, please realize that
the cleancache impact to mm/vfs totals less than 50 lines,
(written by Chris Mason, not me) and these patched lines
have been essentially static since about 2.6.18, and
in all of the versions I've posted. Most of the
cleancache version churn was designing a clean layer
so that those hooks are more broadly useful plus
my inexperience in Linux coding style and the process
for posting patches to lkml. (The layering, plus
some sysfs info and documentation contributes nearly
all of the additional lines of the patch.)

2) Will a lot of Xen users use this? That's sort of a
chicken and egg thing. I have talked to many real
Xen users who would love to use cleancache (actually
Transcendent Memory of which cleancache is the
key part), but they get scared when I tell them it
requires patches that are not upstream yet. Distros
(including Oracle's) are similarly skittish.

At Linux and Xen conferences[1], I've shown a nice performance
improvement on some workloads and negligible worst case
loss. The on-list controversies over cleancache have rarely
involved any performance questions/issues, but I can
revisit the data if it makes a difference on the
decision to merge.

3) GregKH was ready to apply Nitin's zram (actually called
zcache) patch until I pointed out that it was dependent
on cleancache which wasn't yet merged. See:
http://lkml.org/lkml/2010/7/22/491. Due to an internal
API change at v5 (to use exportfs to support fs's where
an inode_t doesn't uniquely represent a file -- with
input and guidance from Andreas Dilger),
zcache needs a few changes and Nitin appears otherwise
occupied right now. If Nitin doesn't get `round to it
and doesn't object, and this is the only barrier to merging
cleancache, I'll be happy to make those changes myself.

I'm separately working on some similar in-kernel compression
ideas, plus the "page-accessible memory" ideas I proposed
for LSF10/MM where, ahem, certain future idiosyncratic fast
solid-state-ish memory technologies are a good match for
cleancache. The core hooks are highly similar to what was
used for Geiger (google Geiger ASPLOS 2006) and I've heard
from several university students that are interested in
researching other ideas built on top of cleancache.
Oh, and at LinuxCon, Anthony Liguori told me he thought
there were at least parts of it that KVM can use.

So, no, this isn't a xen-only thing, nor a one-time
thing. Cleancache is a generic mechanism for grabbing
data from clean pages when they are reclaimed, cacheing
the data in non-kernel-directly-addressable memory, and
avoiding kernel disk reads into file cache when the pages
can be found in cleancache. I just happen to get paid to
work on Xen, so that's where this story started.

4) The on-list lkml patch review process was very helpful
in helping to clean up the cleancache patchset. The
biggest technical hole -- filesystems for which an
inode_t can't uniquely identify a file -- is fixed.
Other technical questions/feedback are summarized
in the commit comments and in the FAQ included with
the patch including, I believe, everything both
on-list and f2f from hch.

A LOT of people have provided review, useful feedback
and questions, and I've tried to be very diligent in
replying to all reviewers. If I've missed any that
would lead anyone to disagree with merging cleancache,
I hope they will re-raise them prior to the next
merge window.

Hope that helps... and I hope I am not sounding defensive.
Thanks again for offering to revisit it.

Thanks,
Dan

[1] For a quick performance summary, see slides 37-39 of http://oss.oracle.com/projects/tmem/dist/documentation/presentations/TranscendentMemoryXenSummit2010.pdf


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