Why are std::fstreams so slow?

Question

I was working on a simple parser and when profiling I observed the bottleneck is in... file read! I extracted very simple test to compare the performance of fstreams

Answer 1

The stream is somehow broken on the MAC, old implementation or setup.

An old setup could cause the FILE to be written in the exe directory and the stream in the user directory, this shouldn't make any difference unless you got 2 disks or other different setting.

On my lousy Vista I get Normal buffer+Uncached:
C++ 201103
FILE* write 4756 ms
FILE* read 5007 ms
fstream write 5526 ms
fstream read 5728 ms

Normal buffer+Cached:
C++ 201103
FILE* write 4747 ms
FILE* read 454 ms
fstream write 5490 ms
fstream read 396 ms

Large Buffer+cached:
C++ 201103
5th run:
FILE* write 4760 ms
FILE* read 446 ms
fstream write 5278 ms
fstream read 369 ms

This shows that the FILE write is faster than the fstream, but slower in read than fstream ... but all numbers are within ~10% of each other.

Try adding some more buffering to your stream to see if that helps.

const int MySize = 1024*1024;
char MrBuf[MySize];
stream.rdbuf()->pubsetbuf(MrBuf, MySize);

The equivalent for FILE is

const int MySize = 1024*1024;
if (!setvbuf ( file , NULL , _IOFBF , MySize )) 
    DieInDisgrace();