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PORTING.md

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Porting snmalloc to a new platform

All of the platform-specific logic in snmalloc is isolated in the Platform Abstraction Layer (PAL). To add support for a new platform, you will need to implement a new PAL for your system.

After version 0.5.2, the PAL is defined by a C++20 Concept. When compiling with C++20 or later, you should get helpful messages about any fields or methods that your PAL is missing.

The PAL must implement the following methods:

[[noreturn]] static void error(const char* const str) noexcept;

Report a fatal error and exit.

static void notify_not_using(void* p, size_t size) noexcept;

Notify the system that the range of memory from p to p + size is no longer in use, allowing the underlying physical pages to recycled for other purposes.

template<ZeroMem zero_mem>
static void notify_using(void* p, size_t size) noexcept;

Notify the system that the range of memory from p to p + size is now in use. On systems that lazily provide physical memory to virtual mappings, this function may not be required to do anything. If the template parameter is set to YesZero then this function is also responsible for ensuring that the newly requested memory is full of zeros.

template<bool page_aligned = false>
static void zero(void* p, size_t size) noexcept;

Zero the range of memory from p to p + size. This may be a simple memset call, but the page_aligned template parameter allows for more efficient implementations when entire pages are being zeroed. This function is typically called with very large ranges, so it may be more efficient to request that the operating system provides background-zeroed pages, rather than zeroing them synchronously in this call

template<bool committed>
static void* reserve_aligned(size_t size) noexcept;
static std::pair<void*, size_t> reserve_at_least(size_t size) noexcept;

Only one of these needs to be implemented, depending on whether the underlying system can provide strongly aligned memory regions. If the system guarantees only page alignment, implement the second. The Pal is free to overallocate based on the platform's desire and snmalloc will find suitably aligned blocks inside the region. reserve_at_least should not commit memory as snmalloc will commit the range of memory it requires of what is returned.

If the system provides strong alignment, implement the first to return memory at the desired alignment. If providing the first, then the Pal should also specify the minimum size block it can provide:

static constexpr size_t minimum_alloc_size = ...;

The PAL is also responsible for advertising the page size:

static constexpr size_t page_size = 0x1000;

This is the granularity at which the PAL is able to mark memory as in-use or not-in-use. The PAL is free to advertise a size greater than the minimum page size if that would be more efficient. When a slab is deallocated, the PAL will be instructed to mark everything after the first page_size bytes as not-in-use and so larger values can lead to more memory overhead.

The page size for any given system depends on both the underlying architecture and the operating system. The value exposed by the PAL may also depend on the Architecture Abstraction Layer (AAL). For example, the Linux PAL advertises 64 KiB on PowerPC but 4 KiB on every other supported architecture:

static constexpr size_t page_size =
	Aal::aal_name == PowerPC ? 0x10000 : 0x1000;

Finally, you need to define a field to indicate the features that your PAL supports:

static constexpr uint64_t pal_features = ...;

These features are defined in the PalFeatures enumeration.

There are several partial PALs that can be used when implementing POSIX-like systems:

  • PALPOSIX defines a PAL for a POSIX platform using no non-standard features.
  • PALBSD defines a PAL for the common set of BSD extensions to POSIX.
  • PALBSD_Aligned extends PALBSD to provide support for aligned allocation from mmap, as supported by NetBSD and FreeBSD.

Each of these template classes takes the PAL that inherits from it as a template parameter. A purely POSIX-compliant platform could have a PAL as simple as this:

class PALMyOS : public PALPOSIX<PALMyOS> {}

Typically, a PAL will implement at least one of the functions outlined above in a more-efficient platform-specific way, but this is not required. Non-POSIX systems will need to implement the entire PAL interface. The Windows, and OpenEnclave and FreeBSD kernel implementations give examples of non-POSIX environments that snmalloc supports.

The POSIX PAL uses mmap to map memory. Some POSIX or POSIX-like systems require minor tweaks to this behaviour. Rather than requiring these to copy and paste the code, a PAL that inherits from the POSIX PAL can define one or both of these (static constexpr) fields to customise the mmap behaviour.

  • default_mmap_flags allows a PAL to provide additional MAP_* flags to all mmap calls.
  • anonymous_memory_fd allows the PAL to override the default file descriptor used for memory mappings.