Is there any difference performance difference between g++-mp-4.8 and g++-4.8?

Question

I'm compiling the same program on two different machines and then running tests to compare performance.

There is a difference in the power of the two machines: one is MacBook Pro with a four 2.3GHz processors, the other is a Dell server with twelve 2.9 GHz processors.

However, the mac runs the test programs in shorter time!!

The only difference in the compilation is that I run g++-mp-4.8 on the machine mac, and g++-4.8 on the other.

EDIT: There is NO parallel computing going on, and my process was the only one run on the server. Also, I've updated the number of cores on the Dell.

EDIT 2: I ran three tests of increasing complexity, the times obtained were, in the format (Dell,Mac) in seconds: (1.67,0.56), (45,35), (120,103). These differences are quite substantial!

EDIT 3: Regarding the actual processor speed, we considered this with the system administrator and still came up with no good reason. Here is the spec for the MacBook processor:

http://ark.intel.com/fr/products/71459/intel-core-i7-3630qm-processor-6m-cache-up-to-3_40-ghz

and here for the server:

http://ark.intel.com/fr/products/64589/Intel-Xeon-Processor-E5-2667-15M-Cache-2_90-GHz-8_00-GTs-Intel-QPI

@Samoth Ok, so the compiler shouldn't make any difference, right? — Plamen
The CPU in the MacBook (not PowerBook, I assume) is simply faster than the CPUs in the Dell server for single-threaded code. For a detailed answer, post the exact models of the systems. CPU performance is not only dependent on frequency - a 2 GHz Intel Core would be much faster than a 2GHz ARM CPU, for example. — Chris
Lol! Yes it's a MacBook. I'm still in 2005 mentally. I've added the details you asked for. We thought about it and it didn't seem to be the right explanation... — Plamen

Ben Voigt Ben Voigt · Accepted Answer · 2014-02-26T17:42:44

I would like to highlight a feature that particularly skews results of single-threaded code on mobile processors:

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Note that while there's a 500 MHz difference in base speed (the question mentioned 2.3 GHz, are we looking at the same CPU?), there's only a 100 MHz difference in single-threaded speed, when Turbo Boost is running at maximum.

The Core-i7 also uses faster DDR than its server counterpart, which normally runs at a lower clock speed with more buffers to support much larger capacities of RAM. Normally the number of channels on the Xeon and difference in L3 cache size makes up for this, but different workloads will make use of cache and main memory differently.

Of course generational improvements can make a difference as well. The significance of Ivy Bridge vs Sandy Bridge varies greatly with application.

A final possibility is that the program runtime isn't CPU-bound. I/O subsystem, speed of GPGPU, etc can affect performance over multiple orders of magnitude for applications that exercise those.

Is there any difference performance difference between g++-mp-4.8 and g++-4.8?

2 Answers