Question about bitmap code
Mitchell Blank Jr
mitch-jemalloc-discuss at bodyfour.com
Mon Nov 5 03:53:13 PST 2012
I've been looking through some of the jemalloc code and had a question
Testing on a 64-bit machine, the arena bin with the most elements is size=8
with 501 items. Since a cacheline is commonly 64 bytes = 512 bits, a
simpler single-level bitmap would seem to win just on memory effects.
It's not clear to me if its advantageous on a branching basis, at least on
64-bit. On average you'd need to look at 4 words to find an unused 8-byte
entry, and fewer for other size bins. I guess on a 32-bit platform, it
might be a different story. Still, I wonder if it is worth touching
another cache line to avoid the comparisons.
Are there cases where the bitmap code is used for more than 512 items in it?
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