We've looked into Hoard. It's fast only if you have a large number of threads that are -constantly- allocating memory, and even then it wastes more memory than a regular heap. The large majority of real-world applications don't do this, but some server applications might. I think Hoard could be useful for that. On the other hand, if performance was critical to you then you'd change your code to not allocate memory so much.

A practical problem we have with Hoard is that it lacks diagnostics. By diagnostics I mean extensive heap reports, fragmentation information, free space, what's the largest block I can safely allocate, give me a callback for every malloc so I can do stats tracking, etc.