Date: Tue, 7 Apr 2020 22:32:29 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: "Expected behavior" for mallocng @ low usage I figured as I tune and prepare to integrate the new malloc, it would be helpful to have a description of what users should see in programs that make only small use of malloc, since this is the "hard case" to get right and since any reports of unexpected behavior would be really useful. For simplicity I'm going to assume 4k pages. If the program makes at least one "very small" (<= 108 bytes) allocation, or at least one allocation of certain other sizes smaller than the page size, you should expect to see a single-page mmap that's divided up something like a buddy allocator. At the top level it consists of 2 2032-byte slots, one of which will be broken into two 1008-byte slots, one of which will be broken into 2 496-byte slots. For "very small" sizes, one of these will in turn be broken up into N equal-sized slots for the requested size class (N=30, 15, 10, 7, 6, 5, or 4). If the page is fully broken up into pairs of 496-byte slots, there are 8 such slots, and only 7 "very small" size classes, so under "very low usage", all such objects should fit in the single page, even if you use a multitude of different sizes. For the next 8 size classes (2 doublings) up to 492 bytes, and depending on divisibility, a group of 2, 3, 5, or 7 slots will be created in a slot of size 496, 1008, or 2032. These can use the same page as the above smaller sizes if there's room available. Above this size, coarse size classing is used at first (until usage reaches a threshold) to avoid allocating a large number of many-slot groups of slightly different sizes that might never be filled. The next doubling consists only of ranges [493,668] and [669,1004], allocated in slots of size 2032 in groups of 3 and 2, respectively; these can use any existing free slot of size 2032. (Once usage has reached a threshold such that adding a group of 5 or 7 slots doesn't cause a dramatic relative increase in total usage, finer-grained size classes will be used.) At higher sizes, groups of slots are not allocated inside a larger slot, but as mmaps consisting of a power-of-two number of pages, which will be split N ways, initially with N=7, 3, 5, or 2 depending on divisibility. As usage increases, so does the N (doubling the number of pages used) which reduces potential for vm space fragmentation and increases the number of slots that can be allocated/freed with fast paths manipulating free masks. At present, coarse size classing is used for all these at first, which can result in significant "waste" but avoids preallocating large (5 or 7) counts of slots that might not ever be used. This is what I'm presently working to improve by allowing direct individual mmaps in cases where they can be efficient. Changes in this area are likely coming soon. Main thing I'm trying to solve still is getting eagar allocation down even further so that small programs don't grow significantly when switching to mallocng.
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