Re: C++ is Slow?

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 5 Feb 2008 07:51:01 -0800 (PST)
Message-ID:
<48326056-e5af-43ed-8755-caadb205e287@e23g2000prf.googlegroups.com>
On Feb 5, 12:19 am, nw <n...@soton.ac.uk> wrote:

I'm constantly confronted with the following two techniques, which I
believe often produce less readable code, but I am told are faster
therefore better. Can anyone help me out with counter examples and
arguments?

1. The STL is slow.

More specifically vector. The argument goes like this:

"Multidimensional arrays should be allocated as large contiguous
blocks. This is so that when you are accessing the array and reach the
end of a row, the next row will already be in the cache. You also
don't need to spend time navigating pointers when accessing the array.
So a 2 dimensional array of size 100x100 should be created like this:

const int xdim=100;
const int ydim=100;

int *myarray = malloc(xdim*ydim*sizeof(int));

and accessed like this:

myarray[xdim*ypos+xpos] = avalue;

Is this argument reasonable? (Sounds reasonable to me, though the
small tests I've performed don't usually show any significant
difference).


It depends.

First, of course, this argument has nothing to do with
std::vector. Whether you use std::vector< int > or malloc in
this case probably won't change anything in time, and will
ensure that the memory is correctly freed in case of an
exception.

Second, whether it is faster to multiply, or to chase pointers,
depends very heavily on the machine architecture. When I did a
test like this on the original Intel 8086, chasing pointers won
hands down.

Third, whatever you do, you should wrap it in a class, so you
can change it later, if it turns out that the implementation
isn't optimal, and is creating a bottleneck.

To me this syntax looks horrible, am I wrong? Is vector the
wrong container to use? (My usual solution would be a
vector<vector<int> >). Would using a valarray help?


Wrap it in a class, and don't worry about it until the profiler
says you have to. At that point, try the different solutions on
the actual target hardware.

2. iostream is slow.

I've encountered this is work recently. I'd not considered it
before, I like the syntax and don't do so much IO generally...
I'm just now starting to process terabytes of data, so it'll
become an issue. Is iostream slow?


Again, it depends on the implementation. At least one person,
in the past, created an implementation which was faster than
stdio. For the most part, however, current implementations are
"fast enough", and implementors haven't bothered improving them,
even when faster implementations exist. Regardless of what you
do, if you're processing terabytes, getting the terabytes
physically into and out of memory will dominate runtimes.

specifically I encountered the following example googling
around. The stdio version runs in around 1second, the iostream
version takes 8seconds. Is this just down to a poor iostream
implementation? (gcc 4 on OS X). Or are there reasons why
iostream is fundamentally slower for certain operations? Are
there things I should be keeping in mind to speed up io?


The standards committee added support for code translation to
filebuf, which unless the implementation is very, very careful,
can slow things down significantly. In the past, Dietmar Kuehl
worked out some meta-programming technique which avoided the
cost as long as the translation was the identity function. I
don't think any implementation uses it, however.

Note that under g++ 2.95.2, which used the classical iostream,
iostream was actually faster than stdio. On the whole, though,
iostream implementations aren't as mature as those of stdio
(which, after all, has been around a long, long time). And I
don't find anywhere near that great of difference on a Sun
Sparc.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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