Why does some Ruby code run twice as fast on a 2.53GHz than on a 2.2GHz Core 2 Duo processor?

Question

(This question attempts to find out why the running of a program can be different on different processors, so it is related to the performance aspect of programming.)

The following program will take 3.6 seconds to run on a Macbook that has 2.2GHz Core 2 Duo, and 1.8 seconds to run on a Macbook Pro that has 2.53GHz Core 2 Duo. Why is that?

That's a bit weird... why doubling the speed when the CPU is only 15% faster in clock speed? I double checked the CPU meter to make sure none of the 2 cores are in 100% usage (so as to see the CPU is not busy running something else). Could it be because one is Mac OS X Leopard and one is Mac OS X Snow Leopard (64 bit)? Both are running Ruby 1.9.2.

p RUBY_VERSION
p RUBY_DESCRIPTION if defined? RUBY_DESCRIPTION
n = 9_999_999
p n

t = 0; 1.upto(n) {|i| t += i if i%3==0 || i%5==0}; p t

The following are just output of the program:

On 2.2GHz Core 2 Duo: (Update: Macbook identifier: MacBook3,1, therefore probably is Intel Core 2 Duo (T7300/T7500))

$ time ruby 1.rb
"1.9.2"
"ruby 1.9.2p0 (2010-08-18 revision 29036) [i386-darwin9.8.0]"
9999999
23333331666668

real  0m3.784s
user  0m3.751s
sys  0m0.021s

2.53GHz Intel Core 2 Duo: (Update: Macbook identifier: MacBookPro5,4, therefore probably is Intel Core 2 Duo Penryn with 3 MB on-chip L2 cache)

$ time ruby 1.rb
"1.9.2"
"ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-darwin10.4.0]"
9999999
23333331666668

real  0m1.893s
user  0m1.809s
sys  0m0.012s

Test run on Windows 7:

time_start = Time.now

p RUBY_VERSION
p RUBY_DESCRIPTION if defined? RUBY_DESCRIPTION

n = 9_999_999
p n

t = 0; 1.upto(n) {|i| t += i if i%3==0 || i%5==0}; p t

print "Took #{Time.now - time_start} seconds to run\n"

Intel Q6600 Quad Core 2.4GHz running Windows 7, 64-bit:

C:\> ruby try.rb
"1.9.2"
"ruby 1.9.2p0 (2010-08-18) [i386-mingw32]"
9999999
23333331666668
Took 3.248186 seconds to run

Intel 920 i7 2.67GHz running Windows 7, 64-bit:

C:\> ruby try.rb
"1.9.2"
"ruby 1.9.2p0 (2010-08-18) [i386-mingw32]"
9999999
23333331666668
Took 2.044117 seconds to run

It is also strange why an i7 with 2.67GHz is slower than a 2.53GHz Core 2 Duo.

Answer 1

why doubling the speed when the CPU is only 15% faster in clock speed?

Quite simply because the performance of computers is determined not solely by CPU clock speed.

Other things to consider are:

• CPU architectures, including e.g. the number of cores on a CPU, or the general ability to run multiple instructions in parallel
• other clock speeds in the system (memory, FSB)
• CPU cache sizes
• installed memory chips (some are faster than others)
• additionally installed hardware (might slow down the system through hardware interruptions)
• different operating systems
• 32-bit vs. 64-bit systems

I'm sure there's a lot more things to add to the above list. I won't elaborate further on the point, but if anyone feels like it, please, feel free to add to the above list.

Answer 2

I suspect that ruby is switching to a arbitrary-precision integer implementation later on the 64 bit os.

Quoting the Fixnum ruby doc:

A Fixnum holds Integer values that can be represented in a native machine word (minus 1 bit). If any operation on a Fixnum exceeds this range, the value is automatically converted to a Bignum.

Here, a native machine word is technically 64 bit, but the interpreter is compiled to run on 32 bit processors.