Date: Sat, 2 May 2020 23:03:43 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Concrete case-by-case examples of mallocng @ low usage On Fri, Apr 10, 2020 at 01:04:07PM -0400, Rich Felker wrote: > The following are tables of initial allocation behavior for all sizes > up through 16k assuming 4k page size. The fine classing figures won't > actually be used for initial allocations, but I've included them for > comparison to illustrate why coarse classes are used. These numbers > were all done by hand independently of the code, so that if actual > behavior differs, it indicates a bug in one or the other. > > > First 16 size classes, no coarse classing: > > 0-12: 2x2032 -> 2x1008 -> 2x496 -> 30x16 > 13-28: 2x2032 -> 2x1008 -> 2x496 -> 15x32 > 29-44: 2x2032 -> 2x1008 -> 2x496 -> 10x48 > 45-60: 2x2032 -> 2x1008 -> 2x496 -> 7x64 > 61-76: 2x2032 -> 2x1008 -> 2x496 -> 6x80 > 77-92: 2x2032 -> 2x1008 -> 2x496 -> 5x96 > 93-108: 2x2032 -> 2x1008 -> 2x496 -> 4x112 > 109-124: 2x2032 -> 2x1008 -> 7x128 > > 125-140: 2x2032 -> 2x1008 -> 7x144 This line is a math error present in the actual code. 7x144 fit in 1008, but not with room for the header of one extra unit (16b). Thus 7x144 get allocated inside a 1168 slot instead. This badly harms memory usage in small programs since the smallest 1168 group is 8k and the group may not get used for anything but satisfying a single malloc(128). For now I'll probably just make it special-case this one as 6x144 instead of 7x, but that wastes space so a better solution should be found. That might be applying coarse size classing so that this size class can start at 14x144 once it switches to fine classing. Rich
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