为什么64位版本的应用程序比32位版本慢得多

Question

In order to solve the 3G (Ubuntu) memory issue (sometimes we do need more memory than 3G), I compiled my app under a 64bit environment to use more memory.

But, my 64bit app is much slower than the 32bit version.

32Bit version is built on a 32 bit machine; 64Bit version is build on a 64 bit machine; both 32Bit and 64Bit versions run on the 64Bit machine in our loading test.

I googled, and some folks said, the unnecessary long type can make the 64bit slower than 32bit, because:

man g++:

   -m64
       Generate code for a 32-bit or 64-bit environment.  The 32-bit environment
       sets int, long and pointer to 32 bits and generates code that runs on any
       i386 system. The 64-bit environment sets int to 32 bits and long and
       pointer to 64 bits and generates code for AMD's x86-64 architecture. For
       darwin only the -m64 option turns off the -fno-pic and -mdynamic-no-pic
       options.

So I changed all my long s to int s, but still same result.

Please advise.

Peter

Edit:

• About memory, both 32 and 64 versions use similar memory, about 1.5 ~ 2.5 GB, and my machine has 9GB physical memory;
• I profiled using OProfile, and for most of the functions, the 64bit version collects more profiling samples than the 32bit version;
• I cannot think of any other bottlenecks, please advise.
• My app is a server, and the loading test was done under a 100 client connections. The server does a lot of computation processing the audio data from the clients.

Answer 1

Profile your app. That will tell you where the slow code is.

Answer 2

1. For the question "why", no one will know the reason without details. You must analyze the profiled result and if there are any problem with the result, post it as a question here.

2. If your app does not need more than 4GB of RAM (1.5~2.5GB in your case), you should try x32 . It's a new ABI that allows for 32-bit pointers in 64-bit environment.