Using black magic to make a fast circular buffer

Re:Using black magic to make a fast circular buffer by vdupras on Saturday January 11, 2025 @11:39AM (#42666990)

Certainly, but by conflating the two, it makes the mmap trick appear better than it is in reality. There are good chances that a "&" change brings a very significant speedup.

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Re:Using black magic to make a fast circular buffer by tom_ on Saturday January 11, 2025 @04:39PM (#42669118)

I updated the author's benchmark: https://gcc.godbolt.org/z/bxEzerjc8

While updating it, I noticed a couple of things:

1. the Makefile doesn't specify -O, which seems to imply -O0, unreasonably flattering the 1-memcpy case because there's only 1 call to a generic memcpy rather than 2. With -O3 the memcpys get inlined

2. make the message size more annoying. The queue size was a multiple of the message size, so the 2-memcpy case was never exercised - and -O3 could see this

(Change 2 actually slightly cancels out change 1, and now the 2-memcpy case is terrible again...)

Who even knows what sort of system the Compiler Explorer executor runs the programs on, but this is what I got from running it, and the relative speeds might be representative:

    %: 0.033716 microseconds per write
    &: 0.031772 microseconds per write
    slow: 0.009715 microseconds per write
    fast: 0.001035 microseconds per write

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