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.
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 :)
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.
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