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Concurrent trie-hash map

Concurrent trie-hash map library -- a general purpose associative array, combining the elements of hashing and radix trie. Highlights:

• Very competitive performance, with logarithmic time complexity on average.
• Lookups are lock-free and inserts/deletes are using fine-grained locking.
• Incremental growth of the data structure (no large resizing/rehashing).
• Optional support for use with shared memory, e.g. memory-mapped file.

The implementation is written in C11 and distributed under the 2-clause BSD license.

NOTE: Delete operations (the key/data destruction) must be synchronised with the readers using some reclamation mechanism. You can use the Epoch-based Reclamation (EBR) library provided HERE.

References (some, but not all, key ideas are based on these papers):

• W. Litwin. Trie Hashing. Proceedings of the 1981 ACM SIGMOD, p. 19-29

• P. L. Lehman and S. B. Yao. Efficient locking for concurrent operations on B-trees. ACM TODS, 6(4):650-670, 1981

API

• thmap_t *thmap_create(uintptr_t baseptr, const thmap_ops_t *ops, unsigned flags)

  • Construct a new trie-hash map. The optional ops parameter can used to set the custom allocate/free operations (see the description of thmap_ops_t below). In such case, the baseptr is the base (start) address of the address space mapping (it must be word-aligned). If ops is set to NULL, then malloc(3) and free(3) will be used as the default operations and baseptr should be set to zero. Currently, the supported flags are:
    • THMAP_NOCOPY: the keys on insert will not be copied and the given pointers to them will be expected to be valid and the values constant until the key is deleted; by default, the put operation will make a copy of the key.
    • THMAP_SETROOT: indicate that the root of the map will be manually set using the thmap_setroot routine; by default, the map is initialised and the root node is set on thmap_create.

• void thmap_destroy(thmap_t *hmap)

  • Destroy the map, freeing the memory it uses.

• void *thmap_get(thmap_t *hmap, const void *key, size_t len)

  • Lookup the key (of a given length) and return the value associated with it. Return NULL if the key is not found (see the caveats section).

• void *thmap_put(thmap_t *hmap, const void *key, size_t len, void *val)

  • Insert the key with an arbitrary value. If the key is already present, return the already existing associated value without changing it. Otherwise, on a successful insert, return the given value. Just compare the result against val to test whether the insert was successful.

• void *thmap_del(thmap_t *hmap, const void *key, size_t len)

  • Remove the given key. If the key was present, return the associated value; otherwise return NULL. The memory associated with the entry is not released immediately, because in the concurrent environment (e.g. multi-threaded application) the caller may need to ensure it is safe to do so. It is managed using the thmap_stage_gc and thmap_gc routines.

• void *thmap_stage_gc(thmap_t *hmap)

  • Stage the currently pending entries (the memory not yet released after the deletion) for reclamation (G/C). This operation should be called before the synchronisation barrier.
  • Returns a reference which must be passed to thmap_gc. Not calling the G/C function for the returned reference would result in a memory leak.

• void thmap_gc(thmap_t *hmap, void *ref)

  • Reclaim (G/C) the staged entries i.e. release any memory associated with the deleted keys. The reference must be the value returned by the call to thmap_stage_gc.
  • This function must be called after the synchronisation barrier which guarantees that there are no active readers referencing the staged entries.

If the map is created using the THMAP_SETROOT flag, then the following functions are applicable:

• void thmap_setroot(thmap_t *thmap, uintptr_t root_offset)

  • Set the root node. The address must be relative to the base address, as if allocated by the thmap_ops_t::alloc routine. Return 0 on success and -1 on failure (if already set).

• uintptr_t thmap_getroot(const thmap_t *thmap)

  • Get the root node address. The returned address will be relative to the base address.

The thmap_ops_t structure has the following members:

• uintptr_t (*alloc)(size_t len)
  • Function to allocate the memory. Must return an address to the memory area of the size len. The address must be relative to the base address specified during map creation and must be word-aligned.
• void (*free)(uintptr_t addr, size_t len)
  • Function to release the memory. Must take a previously allocated address (relative to the base) and release the memory area. The len is guaranteed to match the original allocation length.

Notes

Internally, offsets from the base pointer are used to organise the access to the data structure. This allows user to store the data structure in the shared memory, using the allocation/free functions. The keys will also be copied using the custom functions; if THMAP_NOCOPY is set, then the keys must belong to the same shared memory object.

The implementation was extensively tested on a 24-core x86 machine, see the stress test for the details on the technique.

Caveats

• The implementation uses pointer tagging and atomic operations. This requires the base address and the allocations to provide at least word alignment.

• While the NULL values may be inserted, thmap_get and thmap_del cannot indicate whether the key was not found or a key with a NULL value was found. If the caller needs to indicate an "empty" value, it can use a special pointer value, such as (void *)(uintptr_t)0x1.

Performance

The library has been benchmarked using different key profiles (8 to 256 bytes), set sizes (hundreds, thousands, millions) and ratio between readers and writers (from 60:40 to 90:10). In all cases it demonstrated nearly linear scalability (up to the number of cores). Here is an example result when matched with the C++ libcuckoo library:

Disclaimer: benchmark results, however, depend on many aspects (workload, hardware characteristics, methodology, etc). Ultimately, readers are encouraged to perform their own benchmarks.

Example

Simple case backed by malloc(3), which could be used in multi-threaded environment:

thmap_t *kvmap;
struct obj *obj;

kvmap = thmap_create(0, NULL);
assert(kvmap != NULL);
...
obj = obj_create();
thmap_put(kvmap, "test", sizeof("test") - 1, obj);
...
obj = thmap_get(kvmap, "test", sizeof("test") - 1);
...
thmap_destroy(kvmap);

Packages

Just build the package, install it and link the library using the -lthmap flag.

• RPM (tested on RHEL/CentOS 7): cd pkg && make rpm
• DEB (tested on Debian 9): cd pkg && make deb