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mmap thoughts

P: n/a

I've been thinking about the Python mmap module quite a bit
during the last couple of days. Sadly most of it has just been
thinking ... and reading pages from Google searches ... and
very little of it as been coding.

Mostly it's just academic curiosity (I might be teaching an "overview
of programming" class in a few months, and I'd use Python for most
of the practical examples to cover a broad range of programming
topics, including the whole concept of memory mapping used, on the
one hand, as a file access abstraction and as a form of inter-process
shared memory, on the other).

Initial observations:

* The standard library reference could use some good examples.
At least of those should show use of both anonymous and
named mmap objects as shared memory.
* On Linux (various versions) using Python 2.4.x (for at
least 2.4.4 and 2.4.2) if I create on mmap'ing in one
process, then open the file using 'w' or 'w+' or 'w+b'
in another process then my first process dies with "Bus Error"

This should probably be documented.

(It's fine if I use 'a' (append) modes for opening the file).

* It seems that it's also necessary to extend a file to a given
size before creating a mapping on it. In other words you can't
mmap a newly created, 0-length file.

So it seems like the simplest example of a newly created,
non-anonymous file mapping would be something like:

sz = (1024 * 1024 * 1024 * 2 ) - 1
f=open('/tmp/mmtst.tmp','w+b')
f.seek(sz)
f.write('\0')
f.flush()
mm = mmap.mmap(f.fileno(), sz, mmap.MAP_SHARED)
f.close()

Even creating a zero length file and trying to create a
zero-length mapping on it (with mmap(f.fileno(),0,...)
... with a mind towards using mmap's .resize() method on it
doesn't work. (raises: EnvironmentError: "Errno 22: Invalid
Argument"). BTW: the call to f.flush() does seem to be
required at least in my environments (Linux under 2.6 kernels
various distributions and the aforementioned 2.4.2 and 2.4.4
versions of Python.

* The mmtst.tmp file is "sparse" of course. So its size in
the example above is 2GB ... but the disk usage (du command)
on it is only a few KB (depending on your filesystem cluster
size etc).

* Using a function like len(mm[:]) forces the kernel's filesystem
to return a huge stream of NUL characters. (And might thrash
your system caches a little).

* On my SuSE/Novell 10.1 system, using Python 2.4.2 (their RPM
2.4.2-18) I found that anonymous mmaps would raise an
EnvironmentError. Using the same code on 2.4.4 on my Debian
and Fedora Core 6 system worked with no problem:

anonmm == mmap.mmap(-1,4096,mmap.MAP_ANONYMOUS|mmap.MAP_SHARED)

... and also testing on their 2.4.2-18.5 update with the same
results:

Python 2.4.2 (#1, Oct 13 2006, 17:11:24)
[GCC 4.1.0 (SUSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>import mmap
mm = mmap.mmap(-1,4096, mmap.MAP_ANONYMOUS|mmap.MAP_SHARED)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
EnvironmentError: [Errno 22] Invalid argument
>>>
jadestar@dhcphostname:~uname -a
Linux dhcphostname 2.6.16.13-4-default #1 Wed May 3 ...

* On the troublesome SuSE/Novell box using:

f = open('/dev/zero','w+')
anonmm == mmap.mmap(f.fileno(),4096,
mmap.MAP_ANONYMOUS|mmap.MAP_SHARED)

... seems to work. However, a .resize() on that raises the same
EnvironmentError I was getting before.

* As noted in a few discussions in the past Python's mmap() function
doesn't take an "offset" parameter ... it always uses an offset of 0
(It seems like a patch is slated for inclusion in some future release?)

* On 32-bit Linux systems (or on systems running a 32-bit compilation
of Python) 2GB is, of course, the upper limit of an mmap'ing
The ability to map portions of larger files is a key motivation to
include the previously mentioned "offset" patch.

Other thoughts:

(Going beyond initial observations, now)

* I haven't tested this, but I presume that anonymous|shared mappings
on UNIX can only be shared with child/descendant processes ... since
there's no sort of "handle" or "key" that can be passed to unrelated
processes via any other IPC method; so only fork() based inheritence
will work.

* Another thing I haven't tested, yet: how robust are shared mappings
to adjacent/non-overlapping concurrent writes by multiple processes?
I'm hoping that you can reliably have processes writing updates to
small, pre-assigned, blocks in the mmap'ing without contention issues.

I plan to write some "hammer test" code to test this theory
... and run it for awhile on a few multi-core/SMP systems.

* It would be nice to building something like the threading Queue
and/or POSH support for multi-process support over nothing but
pure Python (presumably using the mmap module to pass serialized
objects around).

* The limitations of Python threading and the GIL for scaling on
SMP and multi-core system are notorious; having a first-class,
and reasonably portable standard library for supporting multi-PROCESS
scaling would be of tremendous benefit now that such MP systems
are becoming the norm.

* There don't seem to be any currently maintained SysV IPC
(shm, message, and semaphore) modules for Python. I guess some
people have managed to hack something together using ctypes;
but I haven't actually read, much less tested, any of that code.

* The key to robust and efficient use of shared memory is going to be
in the design and implementation of locking primitives for using it.

* I'm guessing that some additional IPC method will be required to
co-ordinate the locking --- something like Unix domain sockets,
perhaps. At least I think it would be a practically unavoidable
requirement for unrelated process to share memory.

* For related processes I could imagine a scheme whereby the parent
of each process passes a unique "mailbox" offset to each child
and where that might be used to implement a locking scheme.

It might work something like this:

Master process (parent) creates mapping and initializes
a lock mm[0:4] a child counter and a counter (also at
pre-defined offsets) and a set of mailboxes (set to the
max-number of children, or using a blocks of the mm in a
linked-list).

For each sub-process (fork()'d child) the master increments
the counter, and passes a mailbox offset (counter + current
mailbox offset) to it.

The master then goes into a loop, scanning the mailboxes
(or goes idle with a SIGUSR* handler that scans the mailboxes)

Whenever there are any non-empty mailboxes the master appends
corresponding PIDs to a lock-request queue; then it writes pops
those PIDs and writes them into "lock" offset at mm[0] (perhaps
sending a SIGUSR* to the new lock holder, too).

That process now has the lock and can work on the shared memory
When it's done it would clear the lock and signal the master

All processes have read access to the memory while it's not
locked. However, they have to read the lock counter first,
copy the data into their own address, then verify that the
lock counter has not be incremented in the interim. (All
reads are double-checked to ensure that no changes could
have occurred during the copying).

... there are alot of details I haven't considered about such a
scheme (I'm sure they'll come up if I prototype such a system).

Obvious one could envision more complex data structures which
essentially create a sort of shared "filesystem" in the shared
memory ... where the "master" process is analogous to the filesystem
"driver" for it. Interesting cases for handling dead processes
come up (the master could handle SIGCHLD by clearing locks held by
the dearly departed) ... and timeouts/catatonic processes might be
defined (master process kills the child before forcibly removing the
lock). Recovery of the last of the "master" process might be
possible (define a portion of the shared memory pool that holds the
list of processes who become the new master ... first living one on
that list assume control). But that raises new issues (can't depend
on SIGCHLD in such a scheme checking for living processes would
have to be done via kill 0 calls for example).

It's easy to see how complicated all this could become. The question
is, how simple could we make it and still have something useful?

--
Jim Dennis,
Starshine: Signed, Sealed, Delivered

May 11 '07 #1
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P: n/a
In article <1178924977.507496@smirk>,
"James T. Dennis" <ja******@idiom.comwrote:
* There don't seem to be any currently maintained SysV IPC
(shm, message, and semaphore) modules for Python. I guess some
people have managed to hack something together using ctypes;
but I haven't actually read, much less tested, any of that code.
http://NikitaTheSpider.com/python/shm/
Enjoy =)

--
Philip
http://NikitaTheSpider.com/
Whole-site HTML validation, link checking and more
May 12 '07 #2

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