ThreadLocal and generics - performance

Question

I would like to create some thread local static arrays, using generics. The size of the array depends on the type. I'm trying to do something like:
type LeftoverPool private () =
static let instance = new ThreadLocal<'T[]>(fun () -> Array.zeroCreate (Vector<'T>.Count))
static member Instance = instance.Value
But that constrains T to int.
I've tried making the instance method generic, or the type, but can't get this to work.

Answer 1

From a type perspective, what you're trying to do at the moment really doesn't make sense.
You have a static property Instance but its type its completely ambiguous because it's impossible to resolve the meaning of 'T. In order to do this you need to propagate the type annotation up to the containing type, e.g.:
type LeftoverPool<'T> private () =
static let instance = new ThreadLocal<'T[]>(fun () -> Array.zeroCreate<'T> (Vector<'T>.Count))
static member Instance = instance.Value

Answer 2

Create a generic type, then its static members will be specific for the type. Resharper (for C#) will issue a warning, but here is what you really need.
Also, you need to benchmark ThreadStatic vs ThreadLocal as well, the later is effectively a pool of objects and it my have some lookup overheads compared to thread-static fields that just reside in a special memory region for each thread.
Update
I was going to test it for a while anyway, here is the code and output. ThreadStatic is faster.
[TestFixture]
public class BuffersTests {
public static class LocalBuffers<T> {
[ThreadStatic]
private static T[] _threadStatic;
private static ThreadLocal<T[]> _threadLocal = new ThreadLocal<T[]>(() => new T[10]);
public static T[] ThreadStatic => _threadStatic ?? (_threadStatic = new T[10]);
public static T[] ThreadLocal => _threadLocal.Value;
}
[Test, Ignore]
public void ThreadStaticVsThreadLocal() {
for (int r = 0; r < 10; r++) {
const int count = 100000000;
var sw = new Stopwatch();
sw.Restart();
var sum = 0L;
for (var i = 0; i < count; i++) {
var buffer = LocalBuffers<int>.ThreadStatic;
buffer[0] = 123;
sum += buffer[0] + buffer[1];
}
Assert.IsTrue(sum > 0);
sw.Stop();
Console.WriteLine($"ThreadStatic {sw.ElapsedMilliseconds}");
sw.Restart();
sum = 0L;
for (var i = 0; i < count; i++) {
var buffer = LocalBuffers<int>.ThreadLocal;
buffer[0] = 123;
sum += buffer[0] + buffer[1];
}
Assert.IsTrue(sum > 0);
sw.Stop();
Console.WriteLine($"ThreadLocal {sw.ElapsedMilliseconds}");
Console.WriteLine("---------------------");
}
}
}
ThreadStatic 1286
ThreadLocal 1860
---------------------
ThreadStatic 1312
ThreadLocal 1849
---------------------
ThreadStatic 1334
ThreadLocal 1933
---------------------
ThreadStatic 1390
ThreadLocal 2076
---------------------
ThreadStatic 1438
ThreadLocal 2088
---------------------
ThreadStatic 1295
ThreadLocal 2216
---------------------
ThreadStatic 1317
ThreadLocal 1972
---------------------
ThreadStatic 1380
ThreadLocal 1943
---------------------
ThreadStatic 1410
ThreadLocal 1970
---------------------