Andy Lutomirski wrote:
On 01/16/2014 08:17 PM, Howard Chu wrote:
Andy Lutomirski wrote:
I'm interested in a persistent memory track. There seems to be plenty
of other emails about this, but here's my take:
I'm also interested in this track. I'm not up on FS development these
days, the last time I wrote filesystem code was nearly 20 years ago. But
persistent memory is a topic near and dear to my heart, and of great
relevance to my current pet project, the LMDB memory-mapped database.
In a previous era I also developed block device drivers for
battery-backed external DRAM disks. (My ideal would have been systems
where all of RAM was persistent. I suppose we can just about get there
with mobile phones and tablets these days.)
In the context of database engines, I'm interested in leveraging
persistent memory for write-back caching and how user level code can be
made aware of it. (If all your cache is persistent and guaranteed to
eventually reach stable store then you never need to fsync() a
transaction.)
Hmm. Presumably that would work by actually allocating cache pages in
persistent memory. I don't think that anything like the current XIP
interfaces can do that, but it's certainly an interesting thought for
(complicated) future work.
This might not be pretty in conjunction with something like my
writethrough mapping idea -- read(2) and write(2) would be fine (well,
write(2) might need to use streaming loads), but mmap users who weren't
expecting it might have truly awful performance. That especially
includes things like databases that aren't expecting this behavior.
At the moment all I can suggest is a new mmap() flag, e.g. MAP_PERSISTENT. Not
sure how a user or app should discover that it's supported though.
--Andy
First, I'm not an FS expert. I've never written an FS, touched an
on-disk (or on-persistent-memory) FS format. I have, however, mucked
with some low-level x86 details, and I'm a heavy abuser of the Linux
page cache.
I'm an upcoming user of persistent memory -- I have some (in the form
if NV-DIMMs) and I have an application (HFT and a memory-backed
database thing) that I'll port to run on pmfs or ext4 w/ XIP once
everything is ready.
I'm also interested in some of the implementation details. For this
stuff to be reliable on anything resembling commodity hardware, there
will be some caching issues to deal with. For example, I think it
would be handy to run things like pmfs on top of write-through
mappings. This is currently barely supportable (and only using
mtrrs), but it's not terribly complicated (on new enough hardware) to
support real write-through PAT entries.
I've written an i2c-imc driver (currently in limbo on the i2c list),
which will likely be used for control operations on NV-DIMMs plugged
into Intel-based server boards.
In principle, I could even bring a working NV-DIMM system to the
summit -- it's nearby, and this thing isn't *that* large :)
--Andy
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--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
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