thodg/libgit2/src/alloc.h

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src/alloc.h

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Log

Author	Commit	Date	CI	Message
	9865cd16	2018-03-20T14:23:49		alloc: make memory allocators use function pointers Currently, our memory allocators are being redirected to the correct implementation at compile time by simply using macros. In order to make them swappable at runtime, this commit reshuffles that by instead making use of a global "git_allocator" structure, whose pointers are set up to reference the allocator functions. Like this, it becomes easy to swap out allocators by simply setting these function pointers. In order to initialize a "git_allocator", our provided allocators "stdalloc" and "crtdbg" both provide an init function. This is being called to initialize a passed in allocator struct and set up its members correctly. No support is yet included to enable users of libgit2 to switch out the memory allocator at a global level.
	08b318c0	2018-03-14T10:43:00		stdalloc: extend allocators by file and line Our desired architecture would make allocators completely pluggable, such that users of libgit2 can swap out memory allocators at runtime. While making e.g. debugging easier by not having to do a separate build, this feature can also help maintainers of bindings for libgit2 by tying the memory allocations into the other language's memory system. In order to do so, though, we first need to make our two different pre-existing allocators "stdalloc" and "crtdbg" have the same function signatures, as the "crtdbg" allocators all have an additional file and line argument. This is required to build correct stack traces for debugging memory allocations. As that feature may also be interesting to authors of other applications for debugging libgit2, we now simply add these arguments to our standard allocators. Obviously, this may come with a performance penalty. During some simple benchmarks no real impact could be measured though in contrast to a simple pluggable allocator. The following table summarizes the benchmarks. There were three different builds with our current standard allocator ("standard"), with pluggable authenticators accessed via function pointers ("pluggable") and for pluggable authenticators with file and line being added ("fileline"). Furthermore, there were three scenarios for 100.000.000 allocations of 100B ("small alloc"), 100.000.000 allocations of 100KB ("medium alloc"), and 1.000.000 allocations of 100MB. All results are best of 10 runs. |------------|-------------------|-------------------|-------------------| | build/test | small alloc | medium alloc | big alloc | |------------|-------------------|-------------------|-------------------| | standard | 4539779566, +0.0% | 5912927186, +0.0% | 5166935308, +0.0% | |------------|-------------------|-------------------|-------------------| | pluggable | 4611074505, +1.5% | 5979185308, +1.1% | 5388776352, +4.2% | |------------|-------------------|-------------------|-------------------| | fileline | 4588338192, +1.1% | 6004951910, +1.5% | 4942528135, -4.4% | |------------|-------------------|-------------------|-------------------| As can be seen, there is a performance overhead for pluggable allocators. Furthermore, it can also be seen that there is some big variance between runs, especially in the "big alloc" scenario. This is probably being caused by nondeterministic behaviour in the kernel for dynamic allocations. Still, it can be observed that there should be no real difference between the "pluggable" and "fileline" allocators.
	d2e996fa	2018-03-14T10:36:14		util: extract allocators into its own "alloc.h" header Our "util.h" header is a grabbag of various different functions, where many don't have a clear group they belong to. Our set of allocator functions though can be clearly singled out as a single group of functions that always belongs together. Furthermore, we will need to implement additional functions relating to our allocators subsystem when moving to pluggable allocators. Thus, we should just move these functions into their own "alloc" module.