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A comment on A Deep Introduction to JIT Compilers: JITs are not very Just-in-time. Parent comment

Re:A Deep Introduction to JIT Compilers: JITs are not very J... by littlestymaar on Tuesday July 07, 2020 @06:04PM (#23764623)
You're missing the point here: modifying tcmalloc can give you percentage point improvement, but a copying collector can speed up the allocation by orders of magnitude (the allocation is done in a contiguous memory space, every object adjacent to each other and allicating just becomes a few asm instructions, like 5 or something).

It's like comparing speed of a bike vs airplane: the brand and the quality of the bike doesn't really matter…

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Re:A Deep Introduction to JIT Compilers: JITs are not very J... by jashmatthews on Tuesday July 07, 2020 @06:47PM (#23765072)
People repeat this a lot but it's not entirely true and it's not even close to 10x faster. More like 10%.

Freelist based allocation is actually very nearly as fast as bump pointer allocation. Like 10% if you compare the allocation function in isolation and 1% if you benchmark the whole object allocation path.

http://users.cecs.anu.edu.au/~steveb/pubs/papers/mmtk-sigmet...

Some older malloc implementations like Hoard have bump pointer allocation into empty pages but the newest allocator impementations found that the branch prediction cost of having both paths outweighed the benefits

https://www.microsoft.com/en-us/research/uploads/prod/2019/0...

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Re:A Deep Introduction to JIT Compilers: JITs are not very J... by littlestymaar on Wednesday July 08, 2020 @06:38AM (#23768617)
Reading PDF on mobile is annoying so I can't comment on the content of your first link, but your claim that a bump allocator is no more than 10% faster than a free-list-based one is really suspicious:

- first, as you said, people repeat the opposite a lot, including world class GC experts.

- then, unless I'm missing something, that claim sounds equivalent to claiming than heap allocation is no more than 10% more expensive than stack allocation, which is arguably completely false.

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Re:A Deep Introduction to JIT Compilers: JITs are not very J... by jashmatthews on Wednesday July 08, 2020 @08:49AM (#23769323)
That’s why I linked the paper from JVM GC researchers with the direct quote on freelist vs bump pointer „Finding the right fit is about 10% slower [13] than bump- pointer allocation.”
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