[MethodImpl(MethodImplOptions.AggressiveInlining)]

The JIT compiler logically determines which methods to inline. But sometimes we know better than it does. With AggressiveInlining, we give the compiler a hint. We tell it that the method should be inlined. Actually the only hint we give the compiler is to ignore the size restriction on the method or the property you want to inline. Using this attribute does not guarantee that the method will be inlined. There are 1000 and 1 reasons why it cannot be (being virtual for one thing)

Example

This example benchmarks a method with no attribute, and with AggressiveInlining. The method body contains several lines of useless code. This makes the method large in bytes, so the JIT compiler may decide not to inline it.

And: We apply the MethodImplOptions.AggressiveInlining option to Method2. This is an enum.

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;

class Program
{
    const int _max = 10000000;
    static void Main()
    {
	// ... Compile the methods
	Method1();
	Method2();
	int sum = 0;

	var s1 = Stopwatch.StartNew();
	for (int i = 0; i < _max; i++)
	{
	    sum += Method1();
	}
	s1.Stop();
	var s2 = Stopwatch.StartNew();
	for (int i = 0; i < _max; i++)
	{
	    sum += Method2();
	}
	s2.Stop();
	Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) /
	    _max).ToString("0.00 ns"));
	Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) /
	    _max).ToString("0.00 ns"));
	Console.Read();
    }

    static int Method1()
    {
	// ... No inlining suggestion
	return "one".Length + "two".Length + "three".Length +
	    "four".Length + "five".Length + "six".Length +
	    "seven".Length + "eight".Length + "nine".Length +
	    "ten".Length;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    static int Method2()
    {
	// ... Aggressive inlining
	return "one".Length + "two".Length + "three".Length +
	    "four".Length + "five".Length + "six".Length +
	    "seven".Length + "eight".Length + "nine".Length +
	    "ten".Length;
    }
}
Output

7.34 ns    No options
0.32 ns    MethodImplOptions.AggressiveInlining

We see that with no options, the method calls required seven nanoseconds each. But with inlining specified (with AggressiveInlining), the calls required less than one nanosecond each.

Tip:Consider for a moment all the things you could do with those seven nanoseconds.

Tip 2:If you are scheduling your life based on nanoseconds, please consider reducing your coffee intake.

Advertisements

Cache Consideration in Multi-Threaded Code

In parallel programs is very important to regard cache size and hit rates on a single CPU, but it’s even more important to to consider how the caches of multiple processors/cores interact. Let’s consider a single representative example, which demonstrates the important cache optimisation and emphasizes the value of good tools when it comes to performance optimisation in general.

Let’s first examine the first sequential method, it performs the rudimentary task of summing all the elements in a two-dimensional array of integers and returns the result:


public static int MatrixSumSequential(int [,] matrix)
{
    int sum = 0;
    int rows = matrix.GetUpperBound(0);
    int cols = matrix.GetUpperBound(1);
    for(int i = 0; i < rows; i++)
    {
        for(int j = 0; j < cols; j++)
        {
            sum += matrix[i, j];
        }
    }
    return sum;  
}

We could have used TPL but let’s ignore the huge arsenal of tools TPL provides in our simple example. The following attempt at parallelisation may appear sufficiently reasonable to harvest the fruits of multi-core execution, and even implements a crude aggregation to avoid synchronisation on the shared sum variable:

public static int MatrixSumParallel(int [,] matrix)
{
    int sum = 0;
    int rows = matrix.GetUpperBound(0);
    int cols = matrix.GetUpperBound(1);
    const int THREADS = 4;
    int chunk = row / THREADS;
    int [] localSums = new int[THREADS];
    Threads [] threads = new Threads[THREADS];
    for(int i = = 0; i < THREADS; i++)
    {
        int start = chunk * i;
        int end - chunk * (1 + i);
        int threadNum = i;
        threads[i] = new Thread(() => {
            for(int row = start; row < end; r++)
            {
                for(int col = 0; col < cols; col++)
                {
                    localSums[threadNum] += matrix[row, col];
                }
            }
        });
        threads[i].Start();
        foreach(var thread in threads)
            thread.Join();
    }
    return localSums.Sum();
}

Executing each of the two methods several time on an i7 machine with 6 cores produced the following results for a 2,000 x 2,000 matrix of integers:

* 325ms average for sequential method
* 935ms for the parallel method. Three times as slow as the sequential method!

The obvious question is why?
This is not an example of too fine grained parallelism, because the number of threads is only 4. However if you accept the premise that the problem is somehow the cache related, it would make sense to measure the number of cache misses introduced by the 2 methods above.

The Visual Studio profiler when sampling the execution of each methods with a 2,000 x 2,000 matrix reported 963 exclusive samples in the parallel version and only 659 exclusive samples in the sequential version, the vast majority of samples being on the inner loop line that reads from the matrix.

Why would a line of code writing to localSums introduce so many cache misses in comparison to writing to sum local variable? The answer is that the writes to the shared array invalidate cache lines at other processors/cores, causing every += operating to be a cache miss.
When processor writes to a memory location that is in the cache of another processor/core cache, the hardware causes a cache invalidation, that marks the cache line as invalid. Accessing that line results in a cache miss.

The moral of the story do not blindly introduce parallelisation in a hope that that would also result in performance increase. Always test both versions, you might be surprised at the results!

How IEnumeberable might get you in trouble

Quite often I see a bug where a certain Factory method creates an IEnumeberable of objects.

While initially looking at the code you might not notice it, but that sort of code might lead to excessive creation of instances because of multiple enumeration of objects causing the factory method to create more objects that you expect.

For example let examine this bit of innocent looking code:
Continue reading “How IEnumeberable might get you in trouble”

Blog at WordPress.com.

Up ↑