Date: Mon, 3 Aug 2020 00:22:14 +0800 From: Zhao Zhengyu <zhaozhengyu0@...il.com> To: musl@...ts.openwall.com Subject: Re: When to reclaim pages in __bin_chunk? I read the code of mallocng carefully and found some ideas similar to my current work: For malloc requirements of the “small size”, I split one page into the specified size, and manage them as a group. When free, I give them back to heap together. But at the point of reclaiming physical pages to kernel, I can't get enough inspiration from mallocng. Anyway, I understand the question I asked. Thanks! On Sat, Aug 1, 2020 at 3:24 AM Rich Felker <dalias@...c.org> wrote: > 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 > Content of type "text/html" skipped
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