CPU vs GPU performance comparision with OpenCL

I recently had opportunity to explore an awesome library called OpenCL (Open Computing Language) which enables me to create programs which helps me utilize the computation power of my Graphic Card. I wanted to try out how much faster a normal program (addition of elements to two arrays) would work if I parallize the program using OpenCL.

Using OpenCL

import pyopencl as cl
import numpy
import sys

class CL(object):
    def __init__(self, size=10):
        self.size = size
        self.ctx = cl.create_some_context()
        self.queue = cl.CommandQueue(self.ctx)

    def load_program(self):
		__kernel void part1(__global float* a, __global float* b, __global float* c)
       		unsigned int i = get_global_id(0);

	       c[i] = a[i] + b[i];
        self.program = cl.Program(self.ctx, fstr).build()

    def popCorn(self):
        mf = cl.mem_flags

        self.a = numpy.array(range(self.size), dtype=numpy.float128)
        self.b = numpy.array(range(self.size), dtype=numpy.float128)

        self.a_buf = cl.Buffer(self.ctx, mf.READ_ONLY | mf.COPY_HOST_PTR,
        self.b_buf = cl.Buffer(self.ctx, mf.READ_ONLY | mf.COPY_HOST_PTR,
        self.dest_buf = cl.Buffer(self.ctx, mf.WRITE_ONLY, self.b.nbytes)

    def execute(self):
        self.program.part1(self.queue, self.a.shape, None, self.a_buf, self.b_buf, self.dest_buf)
        c = numpy.empty_like(self.a)
        cl.enqueue_read_buffer(self.queue, self.dest_buf, c).wait()
        print "a", self.a
        print "b", self.b
        print "c", c

if __name__ == '__main__':
    matrixmul = CL(10000000)

Normal program without Optimization

def add(size=10):
    a = tuple([float(i) for i in range(size)])
    b = tuple([float(j) for j in range(size)])
    c = [None for i in range(size)]
    for i in range(size):
        c[i] = a[i]+b[i]

    #print "a", a
    #print "b", b
    print "c", c[:1000]


I compared the performance of both the programs using the tool “time” available in Linux and I noted down the “sys time”
Heres the comparision:


Size Using GPU Without GPU ( i.e. CPU)
100 0.130s 0.030s
1000 0.100s 0.010s
10000 0.130s 0.010s
100000 0.150s 0.050s
1000000 0.170s 0.150s
10000000 0.600s 1.150s

Cleary you see that the GPU outperforms CPU at higher values of size as the program is able to use multiple threads provided by the GPU. At lower values of size there is an appreciable access time associated with GPU, so CPU performs faster.

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7 thoughts on “CPU vs GPU performance comparision with OpenCL

  1. I recently made my first foray into OpenCL: http://ejrh.wordpress.com/2012/01/04/massively-parallel-fractals/

    My program is still significantly slower on the GPU. A benchmark like this is useful to show the effect of using more threads. Perhaps similar benchmarks could help show the overhead of sending the work to the GPU too. (I assumed that this overhead was why it was beneficial to use larger workloads in my program, but now that I think about it, maybe it was because there was not enough work for each thread.)

    • And by “thread” I mean stream processor / cuda core / etc executing. Your actual speedup will be much improved.

  2. Just come across your blog, as someone involved in bringing OpenCL to the world its good to see people picking it up and using it :-) Python is great resource for rapid prototyping of these things and I’m a big fan of Python in general (hoping to attend my first PyCon soon!).

    I think the issue you are hitting here is overcoming the cost of the buffer copy to and from the GPU memory vs the computational effort required to process it. In common with API’s like OpenGL mapping buffers which exist in GPU and CPU space can be expensive as the data has to be copied back and forth across the PCIe bus which is inherently expensive.

    I’d recommend experimenting with making the buffers that contain the arrays __local (or at the least the accumulation array). Also repeat the experiment with varying computational load and a fixed size array so you can feel for the edge of the buffer copy cost vs the computational cost.

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