rpmalloc - General Purpose Memory Allocator

This library provides a public domain cross platform lock free thread caching 16-byte aligned memory allocator implemented in C. The latest source code is always available at https://github.com/mjansson/rpmalloc

Created by Mattias Jansson (@maniccoder) - Discord server for discussions at https://discord.gg/M8BwTQrt6c

Platforms currently supported:

• Windows
• Linux
• MacOS
• iOS
• Android

The code should be easily portable to any platform with atomic operations and an mmap-style virtual memory management API. The API used to map/unmap memory pages can be configured in runtime to a custom implementation and mapping granularity/size.

This library is put in the public domain; you can redistribute it and/or modify it without any restrictions. Or, if you choose, you can use it under the MIT license.

Performance

We believe rpmalloc is faster than most popular memory allocators like tcmalloc, hoard, ptmalloc3 and others without causing extra allocated memory overhead in the thread caches compared to these allocators. We also believe the implementation to be easier to read and modify compared to these allocators, as it is a single source file of ~2200 lines of C code. All allocations have a natural 16-byte alignment.

Contained in a parallel repository is a benchmark utility that performs interleaved unaligned allocations and deallocations (both in-thread and cross-thread) in multiple threads. It measures number of memory operations performed per CPU second, as well as memory overhead by comparing the virtual memory mapped with the number of bytes requested in allocation calls. The setup of number of thread, cross-thread deallocation rate and allocation size limits is configured by command line arguments.

https://github.com/mjansson/rpmalloc-benchmark

Below is an example performance comparison chart of rpmalloc and other popular allocator implementations, with default configurations used.

The benchmark producing these numbers were run on an Ubuntu 16.10 machine with 8 logical cores (4 physical, HT). The actual numbers are not to be interpreted as absolute performance figures, but rather as relative comparisons between the different allocators. For additional benchmark results, see the BENCHMARKS file.

Configuration of the thread and global caches can be important depending on your use pattern. See CACHE for a case study and some comments/guidelines.

Required functions

Before calling any other function in the API, you MUST call the initialization function, either rpmalloc_initialize or rpmalloc_initialize_config, or you will get undefined behaviour when calling other rpmalloc entry point.

Before terminating your use of the allocator, you SHOULD call rpmalloc_finalize in order to release caches and unmap virtual memory, as well as prepare the allocator for global scope cleanup at process exit or dynamic library unload depending on your use case.

Using

The easiest way to use the library is simply adding rpmalloc.[h|c] to your project and compile them along with your sources. The allocator is complete self contained, you are not required to call the init/fini functions from your own code, but can do so in order to initialize and finalize the allocator in specific places or provide your own hooks and/or configuration:

rpmalloc_initialize : Call at process start to initialize the allocator

rpmalloc_initialize_config : Optional entry point to call at process start to initialize the allocator with a custom memory mapping backend, memory page size and mapping granularity.

rpmalloc_finalize: Call at process exit to finalize the allocator

rpmalloc_thread_initialize: Call at each thread start to initialize the thread local data for the allocator

rpmalloc_thread_finalize: Call at each thread exit to finalize and release thread cache back to global cache

rpmalloc_config: Get the current runtime configuration of the allocator

Then simply use the rpmalloc/rpfree and the other malloc style replacement functions. Remember all allocations are 16-byte aligned, so no need to call the explicit rpmemalign/rpaligned_alloc/rpposix_memalign functions unless you need greater alignment, they are simply wrappers to make it easier to replace in existing code.

If you wish to override the standard library malloc family of functions and have automatic initialization/finalization of process and threads, define ENABLE_OVERRIDE to non-zero (default is 1) which will include the malloc.c file in compilation of rpmalloc.c, and then rebuild the library or your project where you added the rpmalloc source. If you compile rpmalloc as a separate library you must make the linker use the override symbols from the library by referencing at least one symbol. The easiest way is to simply include rpmalloc.h in at least one source file and call rpmalloc_linker_reference somewhere - it’s a dummy empty function. For C++ overrides you have to #include <rpnew.h> in at least one source file. The list of libc entry points replaced may not be complete, use libc/stdc++ replacement only as a convenience for testing the library on an existing code base, not a final solution.

For explicit first class heaps, see the rpmalloc_heap*_ API under first class heaps section, requiring RPMALLOC_FIRST_CLASS_HEAPS to be defined to 1 - default is 0, as it imposes a very slight performance hit in deallocation path from an extra conditinal instruction.

Building

To compile as a static library run the configure python script which generates a Ninja build script, then build using ninja. The ninja build produces both a static and a dynamic library named rpmalloc.

By default the dynamic library can be used with LD_PRELOAD/DYLD_INSERT_LIBRARIES to inject in a preexisting binary, replacing any malloc/free family of function calls (when ENABLE_OVERRIDE is defined to 1). This is only implemented for Linux and macOS targets. The list of libc entry points replaced may not be complete, use preloading as a convenience for testing the library on an existing binary, not a final solution.

The latest stable release is available in the master branch. For latest development code, use the develop branch.

Configuration options

Detailed statistics are available if ENABLE_STATISTICS is defined to 1 (default is 0, or disabled), either on compile command line or by setting the value in rpmalloc.c. This will cause a slight overhead in runtime to collect statistics for each memory operation, and will also add 4 bytes overhead per allocation to track sizes.

Integer safety checks on all calls are enabled if ENABLE_VALIDATE_ARGS is defined to 1 (default is 0, or disabled), either on compile command line or by setting the value in rpmalloc.c. If enabled, size arguments to the global entry points are verified not to cause integer overflows in calculations.

Asserts are enabled if ENABLE_ASSERTS is defined to 1 (default is 0, or disabled), either on compile command line or by setting the value in rpmalloc.c.

To include malloc.c in compilation and provide overrides of standard library malloc entry points define ENABLE_OVERRIDE to 1 (this is the default).

To enable support for first class heaps, define RPMALLOC_FIRST_CLASS_HEAPS to 1 (this is the default).

Huge pages

The allocator has support for huge/large pages on Windows, Linux and MacOS. To enable it, pass a non-zero value in the config value enable_huge_pages when initializing the allocator with rpmalloc_initialize_config. If the system does not support huge pages it will be automatically disabled. You can query the status by looking at enable_huge_pages in the config returned from a call to rpmalloc_config after initialization is done.

Quick overview

The allocator uses separate heaps for each thread and partitions memory blocks according to a preconfigured set of size classes, up to 8MiB. Huge blocks above this limit are mapped and unmapped directly. Blocks are allocated from a page of multiple blocks, all of the same size class. Each page is one of three page types, small, medium or large. Each page belongs to an even larger span of pages, each of the same page type.

Implementation details

The allocator is based on a fixed page alignment per page type, and 16 byte block alignment within the page. On Windows this the page alignment is automatically guaranteed up to 64KiB by the VirtualAlloc granularity, and on mmap systems it is achieved by oversizing the mapping and aligning the returned virtual memory address to the required boundaries. By aligning to a fixed size the free operation can locate the header of the memory page without having to do a table lookup by simply masking out the low bits of the address (for 64KiB this would be the low 16 bits).

Memory blocks are divided into the three page types. Small pages have blocks in [0, 4096] bytes, medium blocks (4096, 262144] bytes, and large blocks (262144, 8388608] bytes. The three page types are further divided in block size classes, where small block sizes have a fixed granularity and interval of 16 bytes, and medium and large blocks have a variable interval to limit overhead to a fixed ratio.

Each span belongs to a single heap that owns all containing blocks to are allocated/free. To avoid locks, each span is completely owned by the allocating thread, and all cross-thread deallocations will be deferred to the owner thread through a separate free list per span.

Memory mapping

By default the allocator uses OS APIs to map virtual memory pages as needed, either VirtualAlloc on Windows or mmap on POSIX systems. If you want to use your own custom memory mapping provider you can use rpmalloc_initialize or rpmalloc_initialize_config and pass function pointers to map and unmap virtual memory. These function should reserve and free the requested number of bytes.

The returned memory address from the memory map function MUST be aligned to the system memory page size or the configured page size given during initialization using rpmalloc_initialize_config. Use rpmalloc_config to find the configured system page size for required alignment.

Memory mapping requests are always done in multiples of the memory page size. You can specify a custom page size when initializing rpmalloc with rpmalloc_initialize_config, or pass 0 to let rpmalloc determine the system memory page size using OS APIs. The page size MUST be a power of two.

On macOS and iOS mmap requests are tagged with tag 240 for easy identification with the vmmap tool.

Memory fragmentation

There is no memory fragmentation by the allocator in the sense that it will not leave unallocated and unusable “holes” in the memory pages by calls to allocate and free blocks of different sizes. This is due to the fact that the memory pages allocated for each size class is split up in perfectly aligned blocks which are not reused for a request of a different size. The block freed by a call to rpfree will always be immediately available for an allocation request within the same size class.

However, there is memory fragmentation in the meaning that a request for x bytes followed by a request of y bytes where x and y are at least one size class different in size will return blocks that are at least one memory page apart in virtual address space. Only blocks of the same size will potentially be within the same memory page span.

rpmalloc keeps an “active span” and free list for each size class. This leads to back-to-back allocations will most likely be served from within the same span of memory pages (unless the span runs out of free blocks). The rpmalloc implementation will also use any “holes” in memory pages in semi-filled spans before using a completely free span.

First class heaps

rpmalloc provides a first class heap type with explicit heap control API. Heaps are maintained with calls to rpmalloc_heap_acquire and rpmalloc_heap_release and allocations/frees are done with rpmalloc_heap_alloc and rpmalloc_heap_free. See the rpmalloc.h documentation for the full list of functions in the heap API. The main use case of explicit heap control is to scope allocations in a heap and release everything with a single call to rpmalloc_heap_free_all without having to maintain ownership of memory blocks. Note that the heap API is not thread-safe, the caller must make sure that each heap is only used in a single thread at any given time.

Producer-consumer scenario

Compared to the some other allocators, rpmalloc does not suffer as much from a producer-consumer thread scenario where one thread allocates memory blocks and another thread frees the blocks. In some allocators the free blocks need to traverse both the thread cache of the thread doing the free operations as well as the global cache before being reused in the allocating thread. In rpmalloc the freed blocks will be reused as soon as the allocating thread needs to get new spans from the thread cache. This enables faster release of completely freed memory pages as blocks in a memory page will not be aliased between different owning threads.

Best case scenarios

Threads that keep ownership of allocated memory blocks within the thread and free the blocks from the same thread will have optimal performance.

Threads that have allocation patterns where the difference in memory usage high and low water marks fit within the thread cache thresholds in the allocator will never touch the global cache except during thread init/fini and have optimal performance. Tweaking the cache limits can be done on a per-size-class basis.

Worst case scenarios

Since each thread cache maps spans of memory pages per size class, a thread that allocates just a few blocks of each size class (16, 32, …) for many size classes will never fill each bucket, and thus map a lot of memory pages while only using a small fraction of the mapped memory. However, the wasted memory will always be less than 4KiB (or the configured memory page size) per size class as each span is initialized one memory page at a time. The cache for free spans will be reused by all size classes.

Threads that perform a lot of allocations and deallocations in a pattern that have a large difference in high and low water marks, and that difference is larger than the thread cache size, will put a lot of contention on the global cache. What will happen is the thread cache will overflow on each low water mark causing pages to be released to the global cache, then underflow on high water mark causing pages to be re-acquired from the global cache. This can be mitigated by changing the MAX_SPAN_CACHE_DIVISOR define in the source code (at the cost of higher average memory overhead).

Caveats

VirtualAlloc has an internal granularity of 64KiB. However, mmap lacks this granularity control, and the implementation instead oversizes the memory mapping with configured span size to be able to always return a memory area with the required alignment. Since the extra memory pages are never touched this will not result in extra committed physical memory pages, but rather only increase virtual memory address space.

All entry points assume the passed values are valid, for example passing an invalid pointer to free would most likely result in a segmentation fault. The library does not try to guard against errors!.

Other languages

Johan Andersson at Embark has created a Rust wrapper available at rpmalloc-rs

Stas Denisov has created a C# wrapper available at Rpmalloc-CSharp

License

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to http://unlicense.org

You can also use this software under the MIT license if public domain is
not recognized in your country

The MIT License (MIT)

Copyright © 2017 Mattias Jansson

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.