(forwarding this message, as the reply was off-list)
Kim Hansen wrote:
2009/7/24 Dave Angel <da...@ieee.org>:
It's not a question of how much disk space there is, but how much virtual
space 32 bits can address. 2**32 is about 4 gig, and Windows XP reserves
about half of that for system use. Presumably a 64 bit OS would have a much
larger limit.
Years ago I worked on Sun Sparc system which had much more limited shared
memory access, due to hardware limitations. So 2gig seems pretty good to
me.
There is supposed to be a way to tell the Windows OS to only use 1 gb of
virtual space, leaving 3gb for application use. But there are some
limitations, and I don't recall what they are. I believe it has to be done
globally (probably in Boot.ini), rather than per process. And some things
didn't work in that configuration.
DaveA
Hi Dave,
In the related post I did on the numpy discussions:
http://article.gmane.org/gmane.comp.python.numeric.general/31748
another user was kind enough to run my test program on both 32 bit and
64 bit machines. On the 64 bit machine, there was no such limit, very
much in line with what you wrote. Adding the /3GB option in boot.ini
did not increase the available memory as well. Apparently, Python
needs to have been compiled in a way, which makes it possible to take
advantage of that switch and that is either not the case or I did
something else wrong as well.
I acknowledge the explanation concerning the address space available.
Being an ignorant of the inner details of the implementation of mmap,
it seems like somewhat an "implementation detail" to me that such an
address wall is hit. There may be some good arguments from a
programming point of view and it may be a relative high limit as
compared to other systems but it is certainly at the low side for my
application: I work with data files typically 200 GB in size
consisting of datapackets each having a fixed size frame and a
variable size payload. To handle these large files, I generate an
"index" file consisting of just the frames (which has all the metadata
I need for finding the payloads I am interested in) and "pointers" to
where in the large data file each payload begins. This index file can
be up to 1 GB in size and at times I need to have access to two of
those at the same time (and then i hit the address wall). I would
really really like to be able to access these index files in a
read-only manner as an array of records on a file for which I use
numpy.memmap (which wraps mmap.mmap) such that I can pick a single
element, extract, e.g., every thousand value of a specific field in
the record using the convenient indexing available in Python/numpy.
Now it seems like I have to resort to making my own encapsulation
layer, which seeks to the relevant place in the file, reads sections
as bytestrings into recarrays, etc. Well, I must just get on with
it...
I think it would be worthwhile specifying this 32 bit OS limitation in
the documentation of mmap.mmap, as I doubt I am the only one being
surprised about this address space limitation.
Cheers,
Kim
I agree that some description of system limitations should be included
in a system-specific document. There probably is one, I haven't looked
recently. But I don't think it belongs in mmap documentation.
Perhaps you still don't recognize what the limit is. 32 bits can only
address 4 gigabytes of things as first-class addresses. So roughly the
same limit that's on mmap is also on list, dict, bytearray, or anything
else. If you had 20 lists taking 100 meg each, you would fill up
memory. If you had 10 of them, you might have enough room for a 1gb
mmap area. And your code takes up some of that space, as well as the
Python interpreter, the standard library, and all the data structures
that are normally ignored by the application developer.
BTW, there is one difference between mmap and most of the other
allocations. Most data is allocated out of the swapfile, while mmap is
allocated from the specified file (unless you use -1 for fileno).
Consequently, if the swapfile is already clogged with all the other
running applications, you can still take your 1.8gb or whatever of your
virtual space, when much less than that might be available for other
kinds of allocations.
Executables and dlls are also (mostly) mapped into memory just the same
as mmap. So they tend not to take up much space from the swapfile. In
fact, with planning, a DLL needn't take up any swapfile space (well, a
few K is always needed, realistically).. But that's a linking issue for
compiled languages.
DaveA
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