Date: Fri, 31 Jul 2020 15:23:45 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: When to reclaim pages in __bin_chunk? On Fri, Jul 31, 2020 at 07:22:16PM +0200, Markus Wichmann wrote: > On Fri, Jul 31, 2020 at 08:17:02AM +0800, Zhao Zhengyu wrote: > > Hello, > > > > When chunks are merged, we use "(curr_size + pre_size) ^ pre_size > > > pre_size" to decide whether to reclaim. I think this may be something > > related to performance, but I can’t prove it. I want to know the > > reason. > > > > Thank you! > > > > Zhengyu > > I asked that same question a while ago. For one, this was in the old > malloc code, which is now usually no longer used. For two, this tries to > figure out if adding current size to previous size rolls over into a new > power of two. Usually, curr_size will be small and pre_size will be > large. Therefore, adding the two will not change much about the high > bits of pre_size, so due to the XOR operator, those bits will cancel out > and the result will be smaller than pre_size. However, if the sum does > roll over, then the sum has one bit set that isn't in pre_size and is > larger than all the ones in pre_size. So the XOR can't cancel that bit, > and the result of the XOR is greater than pre_size. > > Fundamentally, it is an optimized version of (a_clz(curr_size + > pre_size) < a_clz(pre_size)). Yes, it's a heuristic at least approximately equivalent to "crossed the next power of two size boundary" to limit the frequency of madvise syscalls when a large free zone keeps getting expanded by adjacent tiny frees. However it does not work very well in practice, and doesn't even mitigate the possibility of continuous syscall load when a repeated malloc/free cycle occurs right at a power-of-two boundary. mallocng handles this kind of thing much better by grouping same-sized allocations and returning them as a group when all are freed, only holding back from doing so if it's observed allocations of this size "bouncing" (repeatedly creating and destroying the same group). Rich
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