Design & roadmap#

pw_allocator: Flexible, safe, and measurable memory allocation

Design of pw::Allocator#

Traditionally, most embedded firmware have laid out their systems’ memory usage statically, with every component’s buffers and resources set at compile time. As systems grow larger and more complex, dynamic allocation provides increasing opportunities to simplify code and improve memory usage by enabling sharing and eliminating large reservations.

As a result, pw_allocator seeks to make dynamic allocation possible without sacrificing too much of the control over memory usage that embedded developers are accustomed to and need. The fundamental design goals of pw_allocator are for allocators to be:

  • Familiar: The interface and its usage should resemble that of C++17’s std::pmr::polymorphic_allocator type.

  • Flexible: A diverse set of allocation strategies should be implementable using allocators.

  • Composable: Allocators should be able to combine and use other allocators.

  • Extensible: Downstream projects should be able to provide their own allocator implementations, and easily integrate them with Pigweed’s.

  • Cost-effective: Projects should be able to include only the allocator behaviors they desire.

  • Observable: Allocators should provide tools and data to reveal how memory is being used.

  • Correcting: Allocators should include features to help uncover memory defects including heap corruption, leaks, use-after-frees, etc.

Differences with C++ polymorphic allocators#

C++17 introduced the <memory_resource> header with support for polymorphic memory resources (PMR), i.e. allocators. This library defines many allocator interfaces similar to those in pw_allocator.

Pigweed has decided to keep pw_allocator distinct from PMR primarily because the latter’s interface requires the use of C++ language features prohibited by Pigweed. In PMR, allocators are expected to throw an exception in the case of failure, and equality comparisons require runtime type identification (RTTI).

Even so, pw_allocator has taken inspiration from the design of PMR, incorporating many of its ideas. Allocator in particular is similar to std::pmr::memory_resource.

This similarity is most evident in the PMR adapter class, PmrAllocator. This adapter allows any Allocator to be used as a std::pmr::polymorphic_allocator with any standard library that can use an allocator. Refer to the guides on how to Use standard library containers.

Forwarding allocator concept#

In addition to concrete allocator implementations, the design of pw_allocator also encourages the use of “forwarding” allocators. These are implementations of the Allocator interface that don’t allocate memory directly and instead rely on other allocators while providing some additional behavior.

For example, the Allocator records various metrics such as the peak number of bytes allocated and the number of failed allocation requests. It wraps another allocator which is used to actually perform dynamic allocation. It implements the allocator API, and so it can be passed into any routines that use dependency injection by taking a generic Allocator parameter.

These “forwarding” allocators are not completely free. At a miniumum, they represent an extra virtual indirection, and an extra function call, albeit one that can often be inlined. Additional behavior-specific code or state adds to their cost in terms of code size and performance. Even so, these “forwarding” allocators can provide savings relative to a “golden hammer”-style allocator that combined all of their features and more. By decomposing allocators into orthogonal behaviors, implementers can choose to pay for only those that they want.

Design of allocator utilities#

In addtion to providing allocator implementations themselves, pw_allocator includes some foundational classes that can be used to implement allocators.

pw::allocator::Block#

Several allocators make use of allocation metadata stored inline with the allocations themselves. Often referred to as a “header”, this metadata immediately precedes the pointer to usable space returned by the allocator. This header allows allocations to be variably sized, and converts allocation into a bin packing problem. An allocator that uses headers has a miniumum alignment matching that of the header type itself.

For pw_allocator, the most common way to store this header is as a Block interfaces. This class is used to construct a doubly-linked list of subsequences of the allocator’s memory region. It was designed with the following features:

  • Templated offset types: Rather than use pointers to the next and previous blocks, Block uses offsets of a templated unsigned integral type. This saves a few bits that can be used for other purposes, since the blocks are always aligned to the block header. It also gives callers the ability to reduce the size of the headers if the allocator’s memory region is sufficently small, e.g. a type of uint16_t could be used if the region could hold no more than 65536 block headers.

  • Splitting and merging: This class centralizes the logic for splitting memory regions into smaller pieces. Usable sub-blocks can either be split from the beginning or end of a block. Additionally, blocks from either end can be split at specified alignment boundaries. This class also provides the logic for merging blocks back together. Together, these methods provide the invariant that a free block is only ever adjacent to blocks in use.

  • Validation and poisoning: On every deallocation, blocks validate their metadata against their neighbors. A block can fail to be validated if it or its neighbors have had their headers overwritten. In this case, it’s unsafe to continue to use this memory and the block code will assert in order make you aware of the problem. Additionally, blocks can “paint” their memory with a known poison pattern that’s checked whenever the memory is next allocated. If the check fails, then some code has written to unallocated memory. Again, the block code will assert to alert you of a “use-after-free” condition.

Tip

In the case of memory corruption, the validation routines themsleves may crash while attempting to inspect block headers. These crashes are not exploitable from a security perspective, but lack the diagnostic information from the usual PW_CHECK macro. Examining a stack trace may be helpful in determining why validation failed.

Allocator metrics#

A common desire for a project using dynamic memory is to clearly understand how much memory is being allocated. However, each tracked metric adds code size, memory overhead, and a per-call performance cost. As a result, pw_allocator is design to allow allocator implementers to select just the metrics they’re interested in.

In particular, the Metrics uses per-metric type traits generated by PW_ALLOCATOR_METRICS_DECLARE to conditionally include the code to update the metrics that are included in its MetricsType template parameter type. A suitable MetricType struct can be created using the PW_ALLOCATOR_METRICS_ENABLE macro, which will only create fields for the enabled metrics.

Using these macros prevents unwanted metrics from increasing either the code size or object size of the metrics adapter, and by extension, TrackingAllocator.

Roadmap#

While the Allocator interface is almost stable, there are some outstanding features the Pigweed team would like to add to pw_allocator:

  • Asynchronous allocators: Determine whether these should be provided, and if so, add them.

  • Additional smart pointers: Determine if pointers like std::shared_ptr, etc., are needed, and if so, add them.

  • Dynamic containers: Provide the concept of allocator equality without using RTTI or typeid. This would allow dynamic containers with their own allocators.

  • Default allocators: Integrate pw_allocator into the monolithic pw_system as a starting point for projects.

Found a bug? Got a feature request? Please create a new issue in our tracker!

Want to discuss allocators in real-time with the Pigweed team? Head over to our Discord!

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