了解.NET内部StringBuilderCache类配置

问题描述

当我查看反编译的.NET程序集以了解一些内部信息时，我注意到多个框架方法使用的有趣的 StringBuilderCache 类:

 内部静态类StringBuilderCache{[ThreadStatic]私有静态StringBuilder CachedInstance；私有const int MAX_BUILDER_SIZE = 360;公共静态StringBuilder Acquire(int容量= 16){如果(容量< = 360){StringBuilder cachedInstance = StringBuilderCache.CachedInstance;if(cachedInstance！=空&&容量< = cachedInstance.Capacity){StringBuilderCache.CachedInstance = null;cachedInstance.Clear();返回cachedInstance；}}返回新的StringBuilder(capacity);}公共静态无效Release(StringBuilder sb){如果(sb.Capacity< = 360){StringBuilderCache.CachedInstance = sb;}}公共静态字符串GetStringAndRelease(StringBuilder sb){字符串结果= sb.ToString();StringBuilderCache.Release(sb);返回结果；}} 

示例用法，例如，我们可以在 string.Format 方法中找到

 公共静态字符串Format(IFormatProvider提供程序，字符串格式，params对象[] args){...StringBuilder stringBuilder = StringBuilderCache.Acquire(format.Length + args.Length * 8);stringBuilder.AppendFormat(provider，format，args);返回StringBuilderCache.GetStringAndRelease(stringBuilder);} 

虽然非常聪明，并且可以肯定我会记得这种缓存模式，但我想知道为什么 MAX_BUILDER_SIZE 这么小?将其设置为2kB，会更好吗?这样可以避免以较小的内存开销创建更大的 StringBuilder 实例.

解决方案

这是一个基于线程的缓存，因此数量较少.最好将参考源用于此类问题，您也将看到注释，其中看起来像(已编辑为适合):

 //值360是在与性能专家讨论时选择的//尽量减少使用litle内存(每个线程)和//仍然涵盖了短暂的StringBuilder创建的大部分内容//VS设计器的启动路径.私有const int MAX_BUILDER_SIZE = 360; 

"VS设计师"有点令人费解.好吧，不是真的，可以肯定的是，这项工作是为了优化Visual Studio而完成的.妮莉·克罗斯(Neelie Kroes)将会在野外度过一天，如果她能找到的话，欧盟将再有10亿美元:

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.

解决方案

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 :)

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