Understanding of .NET internal StringBuilderCache

问题:

When I was looking at decompiled .NET assemblies to see some internals, I've noticed interesting StringBuilderCache class used by multiple framework's methods:

internal static class StringBuilderCache
{
    [ThreadStatic]
    private static StringBuilder CachedInstance;
    private const int MAX_BUILDER_SIZE = 360;
    public static StringBuilder Acquire(int capacity = 16)
    {
        if (capacity <= 360)
        {
            StringBuilder cachedInstance = StringBuilderCache.CachedInstance;
            if (cachedInstance != null && capacity <= cachedInstance.Capacity)
            {
                StringBuilderCache.CachedInstance = null;
                cachedInstance.Clear();
                return cachedInstance;
            }
        }
        return new StringBuilder(capacity);
    }
    public static void Release(StringBuilder sb)
    {
        if (sb.Capacity <= 360)
        {
            StringBuilderCache.CachedInstance = sb;
        }
    }
    public static string GetStringAndRelease(StringBuilder sb)
    {
        string result = sb.ToString();
        StringBuilderCache.Release(sb);
        return result;
    }
}

Example usage we can find for example in string.Format method:

public static string Format(IFormatProvider provider, string format, params object[] args)
{
    ...
    StringBuilder stringBuilder = StringBuilderCache.Acquire(format.Length + args.Length * 8);
    stringBuilder.AppendFormat(provider, format, args);
    return StringBuilderCache.GetStringAndRelease(stringBuilder);
}

While it is quite clever and for sure I will remember about such caching pattern, I wonder why MAX_BUILDER_SIZE is so small? Setting it to, let's set 2kB, wouldn't be better? It would prevent from creating bigger StringBuilder instances with a quite little memory overhead.

回答1:

It is a per-thread cache so a low number is expected. Best to use the Reference Source for questions like this, you'll see the comments as well. Which looks like (edited to fit):

    // The value 360 was chosen in discussion with performance experts as a 
    // compromise between using as litle memory (per thread) as possible and 
    // still covering a large part of short-lived StringBuilder creations on 
    // the startup path of VS designers.
    private const int MAX_BUILDER_SIZE = 360;

"VS designers" is a wee bit puzzling. Well, not really, surely this work was done to optimize Visual Studio. Neelie Kroes would have a field day and the EU would have another billion dollars if she would find out :)

回答2:

Most strings built are probably small, so using a relatively small buffer size will cover most of the operations while not using up too much memory. Consider that there is a thread pool with possibly many threads being created. If every one of them would take up to 2kB for a cached buffer, it would add up to some amount of memory.