function createBenchmark( N ) {
	var x;
	var y;
	var i;

	x = {};
	y = {};
	for ( i = 0; i < N; i++ ) {
		x[ i ] = i;
		y[ i ] = i + 1;
	}
	return benchmark;

	/**
	* Benchmark function.
	*
	* @private
	* @param {Benchmark} b - benchmark instance
	*/
	function benchmark( b ) {
		var keys;
		var i;

		b.tic();
		for ( i = 0; i < b.iterations; i++ ) {
			keys = commonKeys( x, y );
			if ( typeof keys !== 'object' ) {
				b.fail( 'should return an object' );
			}
		}
		b.toc();
		if ( !isStringArray( keys ) && !isEmptyArray( keys ) ) {
			b.fail( 'should return an array of strings' );
		}
		b.pass( 'benchmark finished' );
		b.end();
	}
}
Creates a benchmark function.

@private
@param {PositiveInteger} N - number of properties
@returns {Function} benchmark function