4.43.2 Heap memory (malloc)

SWI-Prolog's memory management is based on the C runtime malloc() function and related functions. The characteristics of the malloc() implementation may affect performance and overall memory usage of the system. For most Prolog programs the performance impact of the allocator is small.^{168Multi-threaded applications may suffer from allocators that do not effectively avoid false sharing that affect CPU cache behaviour or operate using a single lock to provide thread safety. Such allocators should be rare in modern OSes.} The impact on total memory usage can be significant though, in particular for multi-threaded applications. This is due to two aspects of SWI-Prolog memory management:

• The Prolog stacks are allocated using malloc(). The stacks can be extremely large. SWI-Prolog assumes malloc() will use a mechanism that allows returning this memory to the OS. Most todays allocators satisfy this requirement.

• Atoms and clauses are allocated by the thread that requires them, but this memory is freed by the thread running the atom or clause garbage collector (see garbage_collect_atoms/0 and garbage_collect_clauses/0). Normally these run in the thread gc, which means that all deallocation happens in this thread. Notably the ptmalloc implementation used by the GNU C library (glibc) seems to handle this poorly.

Starting with version 8.1.27, SWI-Prolog by default links against tcmalloc when available. Note that changing the allocator can only be done by linking the main executable (swipl) to an alternative library. When embedded (see section 12.4.25) the main program that embeds libswipl must be linked with tcmalloc. On ELF based systems (Linux), this effect can also be achieved using the environment variable LD_PRELOAD:

% LD_PRELOAD=/path/to/libtcmalloc.so swipl ...

SWI-Prolog attempts to detect the currently active allocator and sets the Prolog flag malloc if the detection succeeds. regardless of the malloc implementation, trim_heap/0 is provided.

[det]trim_heap

his predicate attempts to return heap memory to the operating system. There is no portable way of doing so. If the system detects tcmalloc it calls MallocExtension_ReleaseFreeMemory(). If the system detects ptmalloc as provided by the GNU runtime library it calls malloc_trim(). In other cases this predicate simply succeeds. See also trim_stacks/0

4.43.2.1 TCMalloc control predicates

If SWI-Prolog core detects that tcmalloc is the current allocator and provides the following additional predicates.

[nondet]malloc_property(?Property)

True when Property is a property of the current allocator. The properties are defined by the allocator. The properties of tcmalloc are defined in gperftools/malloc_extension.h:^{169Documentation copied from the header.}

’generic.current_allocated_bytes’(-Int)

Number of bytes currently allocated by application.

’generic.heap_size’(-Int)

Number of bytes in the heap (= current_allocated_bytes + fragmentation + freed memory regions).

’tcmalloc.max_total_thread_cache_bytes’(-Int)

Upper limit on total number of bytes stored across all thread caches.

’tcmalloc.current_total_thread_cache_bytes’(-Int)

Number of bytes used across all thread caches.

’tcmalloc.central_cache_free_bytes’(-Int)

Number of free bytes in the central cache that have been assigned to size classes. They always count towards virtual memory usage, and unless the underlying memory is swapped out by the OS, they also count towards physical memory usage.

’tcmalloc.transfer_cache_free_bytes’(-Int)

Number of free bytes that are waiting to be transferred between the central cache and a thread cache. They always count towards virtual memory usage, and unless the underlying memory is swapped out by the OS, they also count towards physical

’tcmalloc.thread_cache_free_bytes’(-Int)

Number of free bytes in thread caches. They always count towards virtual memory usage, and unless the underlying memory is swapped out by the OS, they also count towards physical memory usage.

’tcmalloc.pageheap_free_bytes’(-Int)

Number of bytes in free, mapped pages in page heap. These bytes can be used to fulfill allocation requests. They always count towards virtual memory usage, and unless the underlying memory is swapped out by the OS, they also count towards physical memory usage. This property is not writable.

’tcmalloc.pageheap_unmapped_bytes’(-Int)

Number of bytes in free, unmapped pages in page heap. These are bytes that have been released back to the OS, possibly by one of the MallocExtension "Release" calls. They can be used to fulfill allocation requests, but typically incur a page fault. They always count towards virtual memory usage, and depending on the OS, typically do not count towards physical memory usage.

[det]set_malloc(+Property)

Set properties described in malloc_property/1. Currently the only writable property is tcmalloc.max_total_thread_cache_bytes. Setting an unknown property raises a domain_error and setting a read-only property raises a permission_error exception.

[semidet]thread_idle(:Goal, +Duration)

Indicates to the system that the calling thread will idle for some time while calling Goal as once/1. This call releases resources to the OS to minimise the footprint of the calling thread while it waits. Despite the name this predicate is always provided, also if the system is not configured with tcmalloc or is single threaded. Duration is one of

short

Calls trim_stacks/0 and, if tcmalloc is used, calls MallocExtension_MarkThreadTemporarilyIdle() which empties the thread's malloc cache but preserves the cache itself.

long

Calls garbage_collect/0 and trim_stacks/0 and, if tcmalloc is used, calls MallocExtension_MarkThreadIdle() which releases all thread-specific allocation data structures.