d900e29424
* Remote dealloc refactor. * Improve remote dealloc Change remote to count down to 0, so fast path does not need a constant. Use signed value so that branch does not depend on addition. * Inline remote_dealloc The fast path of remote_dealloc is sufficiently compact that it can be inlined. * Improve fast path in Slab::alloc Turn the internal structure into tail calls, to improve fast path. Should be no algorithmic changes. * Refactor initialisation to help fast path. Break lazy initialisation into two functions, so it is easier to codegen fast paths. * Minor tidy to statically sized dealloc. * Refactor semi-slow path for alloc Make the backup path a bit faster. Only algorithmic change is to delay checking for first allocation. Otherwise, should be unchanged. * Test initial operation of a thread The first operation a new thread takes is special. It results in allocating an allocator, and swinging it into the TLS. This makes this a very special path, that is rarely tested. This test generates a lot of threads to cover the first alloc and dealloc operations. * Correctly handle reusing get_noncachable * Fix large alloc stats Large alloc stats aren't necessarily balanced on a thread, this changes to tracking individual pushs and pops, rather than the net effect (with an unsigned value). * Fix TLS init on large alloc path * Add Bump ptrs to allocator Each allocator has a bump ptr for each size class. This is no longer slab local. Slabs that haven't been fully allocated no longer need to be in the DLL for this sizeclass. * Change to a cycle non-empty list This change reduces the branching in the case of finding a new free list. Using a non-empty cyclic list enables branch free add, and a single branch in remove to detect the empty case. * Update differences * Rename first allocation Use needs initialisation as makes more sense for other scenarios. * Use a ptrdiff to help with zero init. * Make GlobalPlaceholder zero init The GlobalPlaceholder allocator is now a zero init block of memory. This removes various issues for when things are initialised. It is made read-only to we detect write to it on some platforms.
42 lines
1.9 KiB
Markdown
42 lines
1.9 KiB
Markdown
# Difference from published paper
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This document outlines the changes that have diverged from
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[the published paper](snmalloc.pdf) on `snmalloc`.
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1. Link no longer terminates the bump-free list. The paper describes a
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complex invariant for how the final element of the bump-free list can
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also be the link node.
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We now have a much simpler invariant. The link is either 1, signifying
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the block is completely full. Or not 1, signifying it has at least one
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free element at the offset contained in link, and that contains the DLL
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node for this sizeclass.
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The bump-free list contains additional free elements, and the remaining
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bump allocated space.
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The value 1, is never a valid bump allocation value, as we initially
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allocate the first entry as the link, so we can use 1 as the no more bump
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space value.
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2. Separate Bump/Free list. We have separate bump ptr and free list. This
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is required to have a "fast free list" in each allocator for each
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sizeclass. We bump allocate a whole os page (4KiB) worth of allocations
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in one go, so that the CPU predicts the free list path for the fast
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path.
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3. Per allocator per sizeclass fast free list. Each allocator has an array
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for each small size class that contains a free list of some elements for
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that sizeclass. This enables a very compressed path for the common
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allocation case.
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4. We now store a direct pointer to the next element in each slabs free list
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rather than a relative offset into the slab. This enables list
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calculation on the fast path.
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5. There is a single bump-ptr per size class that is part of the
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allocator structure. The per size class slab list now only contains slabs
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with free list, and not if it only has a bump ptr.
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[2-4] Are changes that are directly inspired by
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(mimalloc)[http://github.com/microsoft/mimalloc]. |