[[Topic diverted from R-help]]
>>>>> "VK" == Vadim Kutsyy <[EMAIL PROTECTED]>
>>>>> on Fri, 01 Aug 2008 07:35:01 -0700 writes:
VK> Martin Maechler wrote:
>>
VK> The problem is in array.c, where allocMatrix check for
VK> "if ((double)nrow * (double)ncol > INT_MAX)". But why
VK> itn is used and not long int for indexing? (max int is
VK> 2147483647, max long int is 9223372036854775807)
>> Well, Brian gave you all info:
>> ( ?Memory-limits )
VK> exactly, and given that most modern system used for
VK> computations (i.e. 64bit system) have long int which is
VK> much larger than int, I am wondering why long int is not
VK> used for indexing (I don't think that 4 bit vs 8 bit
VK> storage is an issue).
Well, fortunately, reasonable compilers have indeed kept
'long' == 'long int' to mean 32-bit integers
((less reasonable compiler writers have not, AFAIK: which leads
of course to code that no longer compiles correctly when
originally it did))
But of course you are right that 64-bit integers
(typically == 'long long', and really == 'int64') are very
natural on 64-bit architectures.
But see below.
>> Did you really carefully read ?Memory-limits ??
VK> Yes, it is specify that 4 bit int is used for indexing
VK> in all version of R, but why? I think 2147483647
VK> elements for a single vector is OK, but not as total
VK> number of elements for the matrix. I am running out of
VK> indexing at mere 10% memory consumption.
If you have too large a numeric matrix, it would be larger than
2^31 * 8 bytes ~= 2^34 / 2^20 ~= 16'000 Megabytes.
If that is is 10% only for you, you'd have around 160 GB of
RAM. That's quite a impressive.
I agree that it is at least in the "ball park" of what is
available today.
[........]
VK> PS: I have no problem to go and modify C code, but I am
VK> just wondering what are the reasons for having such
VK> limitation.
Compatibility for one:
Note that R objects are (pointers to) C structs that are
"well-defined" platform independently, and I'd say that this
should remain so.
Consequently 64ints (or another "longer int"), would have to be
there "in R", also on 32bit platforms. That may well be feasible,
but it would double the size of quite a few objects.
I think what you are implicitly proposing is that
we'd want 64-bit integer as an R-level type, and that are R
would use (and/or coerce to it from 'int32') for indexing
everywhere.
But more importantly, all (or very much of) the currently
existing C- and Fortran-code (called via .Call(), .C(), .Fortran)
would also have to be able to deal with the "longer ints".
One of the last times this topic came up (within R-core),
we found that for all the matrix/vector operations,
we really would need versions of BLAS / LAPACK that would also
work with these "big" matrices, ie. such a BLAS/Lapack would
also have to internally use "longer int" for indexing.
At that point in time, we had decied we would at least wait to
hear about the development of such BLAS/LAPACK libraries.
Interested to hear other opinions / get more info on this topic.
I do agree that it would be nice to get over this limit within a
few years.
Martin
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