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Reading in an entire file at once in C++, part 2

Last week I discussed 6 different methods on how to quickly get an entire file into a C++ string.

The conclusion was that the straight forward method was the ideal method, this was verified across several compilers. Since then, people asked me a number of questions.

What if I want to use a vector instead of a string, are the speeds different?
Forget C++ containers, what about directly into a manually allocated buffer?
What about copying into a buffer via mmap?
What do these various cases say about compilers or their libraries? Can this indicate what they're good or bad at compiling?

So to establish some baseline numbers, I created an application which reads the same files the same amount of times using the fastest method possible. It directly creates a buffer aligned to the partition's blocking factor, informs the OS to do the fastest possible sequential read, and then reads the file directly into the buffer. I also tested the suggested method to mmap a file, and copy the contents from there to an allocated buffer. The times achieved for these were 5 and 9 respectively. The latter is slower because of the extra memcpy() required, you want to read directly into your destination buffer whenever possible. The fastest time I now achieved should more or less be the fastest theoretical limit I'm able to get with my hardware. Both GCC and LLVM got the exact same times. I did not test with VC++ here as it doesn't support POSIX 2008.

Now regarding our 6 methods from last time, all of them except the Rdbuf method are possible with std::vector, since there is no std::vector based std::istream.

An interesting thing to note is that C++ string implementations vary greatly from implementation to implementation. Some offer optimizations for very small strings, some offer optimizations for frequent copies, by using reference counting. Some always ensure the string ends with a 0 byte so you can immediately pass them as a C string. In this latter case, operations which operate on strings as a range are rather quick, as the data is copied, then a 0 is appended. Whereas a loop which constantly pushes bytes on the back will have to needlessly set the extra trailing byte to 0 each time. Vector implementations on the other hand don't need to worry about a trailing 0 byte, and generally don't try to internally use all kinds of elaborate storage methods. So if std::vector works for you, you may want to use that.

Let's review times for the 5 applicable methods with our various compilers.

GCC 4.6 with a vector:

Method	Duration
C	23.5
C++	22.8
Iterator	73
Assign	81.8
Copy	68

Whereas with a string:

Method	Duration
C	24.5
C++	24.5
Iterator	64.5
Assign	68
Copy	63

We see that with a vector, the basic methods became a bit faster, but interestingly enough, the others got slower. However, which methods are superior to the others have remained the same.

Now for LLVM 3 with a vector:

Method	Duration
C	8
C++	8
Iterator	860
Assign	1328
Copy	930

Versus for string:

Method	Duration
C	7.5
C++	7.5
Iterator	110
Assign	102
Copy	97

With LLVM, everything is slower with a vector, and for the more complex solutions, much much slower. There's two interesting things we can see about LLVM though. For more straight forward logic, their compiler's optimizations are extremely smart. The speeds approach the theoretical best. I did some profiling on GCC and LLVM, as they're using the same C and C++ libraries, and found that in the straight C/C++ methods for my test program, GCC made 300 memory allocations, but LLVM made only 200. LLVM apparently is smart enough to see inside the various allocations, skip the ones that aren't needed, and place the data directly into the output buffer. But for complex code, LLVM's optimizations aren't that great. In the case of vectors and iterators, downright awful. Someone should file some bug reports with them.

Now for Visual C++ 2010 using vector:

Method	Duration
C	17.8
C++	18.7
Iterator	180.6
Assign	159.5
Copy	165.6

And string:

Method	Duration
C	16.5
C++	20.4
Iterator	224.4
Assign	222.8
Copy	320

We see here that the Copy method, which uses push_back() got a huge performance improvement. This seems to indicate that the STL implementation adds a 0 at the end of each operation, especially push_back(), instead of just when c_str() is called. Otherwise, string is faster.

It's also sad to see that GCC while winning all the cases where iterators were involved, was significantly slower in all the straight forward cases. This seems to indicate that GCC has the smartest optimizations, but fails to optimize well when the logic is straightforward. Someone should look into that.

It seems if you're trying to hold a collection of bytes, or whatever your wchar_t is, but don't care about the specialties of any particular container, as long as you don't push_back() a lot, string seems to be faster.

Finally, here's a table of all the compilers and methods I tested ordered by speed:

Method	Duration
POSIX	5
LLVM 3.0 s C/C++	7.5
LLVM 3.0 v C/C++	8
MMAP	9
VC 2010 s C	16.5
VC 2010 v C	17.8
VC 2005 s C	18.3
VC 2010 v C++	19.7
VC 2010 s C++	20.4
VC 2005 s C++	21
GCC 4.6 v C++	22.8
GCC 4.6 v C	23.5
VC 2005 v C	24
GCC 4.6 s C/C++	24.5
VC 2005 v C++	26
LLVM 3.0 s Rdbuf	31.5
GCC 4.6 s Rdbuf	32.5
GCC 4.6 s Copy	63
GCC 4.6 s Iterator	64.5
GCC 4.6 s Assign	68
GCC 4.6 v Copy	68
GCC 4.6 v Iterator	73
GCC 4.6 v Assign	81.8
LLVM 3.0 s Copy	97
LLVM 3.0 s Assign	102
LLVM 3.0 s Iterator	110
VC 2010 v Assign	159.5
VC 2010 v Copy	165.6
VC 2005 v Copy	172
VC 2010 s Rdbuf	176.2
VC 2010 v Iterator	180.6
VC 2005 s Rdbuf	199
VC 2005 s Iterator	209.3
VC 2005 s Assign	221
VC 2010 s Assign	222.8
VC 2010 s Iterator	224.4
VC 2010 s Copy	320
VC 2005 v Iterator	370
VC 2005 v Assign	378
VC 2005 s Copy	483.5
LLVM 3.0 v Iterator	860
LLVM 3.0 v Copy	930
LLVM 3.0 v Assign	1328

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