I've heard that the Hotspot JVM uses a bump allocator but don't know the details. I'm sure it's heavily optimized though, so I'm curious about how this compares.
For most specific purposes, a specialized bump allocator can be significantly more optimized.
Hard-coding the alignment simplifies many questions (especially if you can guarantee the allocation size being a multiple of the alignment, at which point it becomes a complete non-issue).
And if you have an upper bound on the requested allocation size, the integer overflow checks can be dropped too (e.g. in a Java "new int[n]", "n" is an int, which has a max value of 2^31-1, which can safely be added to a bump pointer, provided it's further than that away from the address space end (which it's gonna be due to the kernel reserving the upper half of the address space)).
And for constant-size objects, your entire bump allocator can become just
if (ptr > precomputedEndMinusObjectSize) fallback();
result = ptr;
ptr += size;
Comments
I've heard that the Hotspot JVM uses a bump allocator but don't know the details. I'm sure it's heavily optimized though, so I'm curious about how this compares.
For most specific purposes, a specialized bump allocator can be significantly more optimized.
Hard-coding the alignment simplifies many questions (especially if you can guarantee the allocation size being a multiple of the alignment, at which point it becomes a complete non-issue).
And if you have an upper bound on the requested allocation size, the integer overflow checks can be dropped too (e.g. in a Java "new int[n]", "n" is an int, which has a max value of 2^31-1, which can safely be added to a bump pointer, provided it's further than that away from the address space end (which it's gonna be due to the kernel reserving the upper half of the address space)).
And for constant-size objects, your entire bump allocator can become just
I believe HotSpot can move allocations out of the arena when garbage collection happens, which means fragmentation is less of an issue.
Here's an article[0] with some information on Hotspot's bump allocators.
[0] https://shipilev.net/jvm/anatomy-quarks/4-tlab-allocation/