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.
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
Whatever you do, there's another advantage to the mmap trick: if the items are variable sizes, they never need to be split. (Split items are often a pain to deal with.) The circularity becomes the buffer's problem alone, and consumers don't have to handle it.
Another article about this here: https://fgiesen.wordpress.com/2012/07/21/the-magic-ring-buff... - see also the comments.