[webkit-help] What is the purpose of WTF::OSAllocator and WTF::fastMalloc in Webkit?

[fdgfd gfdgfdg]

There are two ways webkit gets memory using mmap from the operating system,
one is WTF::OSAllocator and the second one is WTF::fastMalloc which seems
to rely on tcmalloc to get memory from os. I'm using qt bindings. What is
the purpose of each of them ?
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