Our First Foray into Pools

The Shared ArrayPool

For this post we are going to jump straight into looking at some pools. First up is one in .Net called “ArrayPool”

The basic methods and use looks something like this

var pool = new System.Buffers.ArrayPool<byte>.Shared;
var myBuffer = pool.Rent(minBufferSize: 1024);

// some code to do cool things with a buffer

pool.Return(myBuffer);

This might be the first, and probably the easiest way for most people to get into pooling. In fact if you use the “defaults” on pipelines and internally in things like SslStream you are already using this pool. So let’s dig into the properties of this pool to use it as a baseline for future pool discussions.

The API

First up you get back an array. If you don’t know anything about the new Span or Memory types now would be a good time to go and get some in depth understanding of these types. Stephen Toub has a great post about this [here](https://msdn.microsoft.com/en-us/magazine/mt814808.aspx). I am going to assume you understand those types, if you don't go and read that post.

So we have an array, if we want to use sections or slices of that array we can put it inside a Span or Memory and if the method can only take an array we can use that as well (we could also use the older ArraySegment but we are going to ignore that for the newer types). So we have solid flexibility however, we need to ensure that the array is returned to the pool. Our lower level code must be the one returning the array because any method we pass the array into doesn't have any understanding of the lifetime of the array or where it came from.

So say we have some code that takes data from the network, does some processing and then hands that array off to some application that may take a while, we need to either copy that data inside the application code and return the buffer straight away to the networking code, hard-code the ArrayPool into our application code (meaning that we are now dependent on the one implementation of a pool), or finally we could wrap the array in a class or struct and have that type understand where the array should be returned.

This would allow decoupling the buffer and pool implementation from our application. So if we would develop something like this what might it look like?

Would we make it implement IDisposable and return the buffer on dispose to be reused? It would probably contain a Memory rather than an array to give us more flexibility around the source of the memory (if we wanted to use unmanaged memory or chunks of an array which we will get into in later articles). Well helpfully there are classes in .NET Core 2.1 that already provides this called IOwnedMemory and we will get to that in later pools. But keep in mind that this ArrayPool returns an array that has no ownership semantics.

Inside the sausage factory

The default shared buffer uses a bucketing principle. Because you can ask for various sized buffers and it is unknown ahead of time what these might be it has a number of buckets of various sized buffers available for us. So first we can take a look at the defaults for the number of buckets and the maximum size for our bufferS.

/// <summary>The default maximum length of each array in the pool (2^20).</summary>
private const int DefaultMaxArrayLength = 1024 * 1024;
/// <summary>The default maximum number of arrays per bucket that are available for rent.</summary>
private const int DefaultMaxNumberOfArraysPerBucket = 50;

The shared pool uses the default settings. So we can see 1mb is the largest array and we have 50 arrays per bucket. This means if we try to take 51 buffers in a single bucket and return them all then the 51st buffer will be left for the garbage collector to clean up. This is something to keep in mind if you plan on having a number of buffers outside the pool at anyone time. Also remember that you might not be the only one using the pool so that number could be a lot lower.

Next we can take a look at the logic inside the pool for taking a buffer.

int index = Utilities.SelectBucketIndex(minimumLength)
if (index < _buckets.Length)
{
// Search for an array starting at the 'index' bucket. If the bucket is empty, bump up to the next higher bucket and try that one, but only try at most a few buckets
const int MaxBucketsToTry = 2;
int i = index;
do
{
    // Attempt to rent from the bucket.  If we get a buffer from it, return it.
    buffer = _buckets[i].Rent();
    if (buffer != null) return buffer;
} while (++i < _buckets.Length && i != index + MaxBucketsToTry);

// The pool was exhausted for this buffer size.  Allocate a new buffer with a size corresponding to the appropriate bucket.
buffer = new T[_buckets[index]._bufferLength];

I have edited the code for brevity (removing logging and some spacing), however the basic flow is

  1. Find the bucket we should get the buffer from
  2. Check if there is a free buffer in this bucket
  3. If there is no free buffer check in the next highest bucket and repeat 2 (for a max of 2 buckets)
  4. If no buffer is available still then return a new buffer

So far so good, we can see that it is going to be more expensive than just a “new” operation however we knew that already. Now we can take a look inside the buckets Rent method.

bool lockTaken = false, allocateBuffer = false;
try
{
    _lock.Enter(ref lockTaken);
    if (_index < buffers.Length)
    {
        buffer = buffers[_index];
        buffers[_index++] = null;
        allocateBuffer = buffer == null;
    }
}
finally
{
    if (lockTaken) _lock.Exit(false);
}

if (allocateBuffer) buffer = new T[_bufferLength];
return buffer;

So here we take a spin lock for a short time, check if there is a free buffer in the bucket and if that slot is empty (we have never allocated for it yet) we will just create a new buffer.

There is nothing wrong with this method however you should be aware of a few things about this pool and why it may or may not be the best approach for your application.

  1. The max bucket size is fixed
  2. The high water mark for each buffer is fixed
  3. There are buckets of varying sizes
  4. Multiple pieces of code outside your control can use it in different ways
  5. It only provides Arrays with no solid return semantics
  6. There is no zeroing of data
  7. There is bucket stealing from up to 2 categories higher
  8. Every array is a separate object per buffer, and all allocated independently

There are two points I want you to consider here. One is that other pieces of code could be using the buffers in different ways. This is important because if some code is aging the buffers (holding them until they are say Gen2) and another piece of code is taking many buffers out quickly and then returning them and you are hitting that high watermark of 30 buffers you could end up generating a lot of Gen2 objects and may gain very little benefit from pooling.

The upside to the above problem is that if you have many parts of the system using the shared pool in a minimal way you will keep the total memory use lower than each having their own pool.

The last point I want to make before we move onto other pools is the final bullet point. This means that for every array we have to allocate an object. This can be problematic for very small buffers, but also for fragmentation of the heap. The reason? Many applications of pooled buffers end up at some native API such as Sockets. When this happens the buffers end up being pinned, during this time they cannot be moved by the GC. If you have many small buffers spread out in your memory footprint that are being pinned often you can end up fragmenting your heap.

This is the end of part 2, in the next part we will take a quick look at MemoryPool and then move onto "The slab allocator" which will show how some of the concerns of using the ArrayPool in high volume low latency scenarios can be improved.

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