2 * Written by Josh Dybnis and released to the public domain, as explained at
3 * http://creativecommons.org/licenses/publicdomain
5 * C implementation of Cliff Click's lock-free hash table from
6 * http://www.azulsystems.com/events/javaone_2008/2008_CodingNonBlock.pdf
7 * http://sourceforge.net/projects/high-scale-lib
9 * Note: This is code uses synchronous atomic operations because that is all that x86 provides.
10 * Every atomic operation is also an implicit full memory barrier. The upshot is that it simplifies
11 * the code a bit, but it won't be as fast as it could be on platforms like SPARC that provide
12 * weaker operations which would still do the job.
20 #define COPIED_VALUE (-1)
21 #define TOMBSTONE STRIP_TAG(COPIED_VALUE)
23 #define ENTRIES_PER_BUCKET (CACHE_LINE_SIZE/sizeof(entry_t))
24 #define ENTRIES_PER_COPY_CHUNK (ENTRIES_PER_BUCKET * 2)
25 #define MIN_SCALE (__builtin_ctz(ENTRIES_PER_BUCKET) + 2) // min 4 buckets
26 #define MAX_BUCKETS_TO_PROBE 250
28 #define GET_PTR(x) ((string_t *)((x) & MASK(48))) // low-order 48 bits is a pointer to a string_t
30 typedef struct ht_entry {
35 typedef struct string {
40 typedef struct hash_table_i {
41 volatile entry_t *table;
42 hash_table_t *ht; // parent ht;
43 struct hash_table_i *next;
44 struct hash_table_i *next_free;
47 int count; // TODO: make these counters distributed
48 int num_entries_copied;
52 static int hti_copy_entry
53 (hash_table_i_t *ht1, volatile entry_t *e, uint32_t e_key_hash, hash_table_i_t *ht2);
55 // Choose the next bucket to probe using the high-order bits of <key_hash>.
56 static inline int get_next_ndx(int old_ndx, uint32_t key_hash, int ht_scale) {
57 int incr = (key_hash >> (32 - ht_scale));
58 incr += !incr; // If the increment is 0, make it 1.
59 return (old_ndx + incr) & MASK(ht_scale);
64 // A key is made up of two parts. The 48 low-order bits are a pointer to a null terminated string.
65 // The high-order 16 bits are taken from the hash of that string. The bits from the hash are used
66 // as a quick check to rule out non-equal keys without doing a complete string compare.
67 static inline int ht_key_equals (uint64_t a, uint32_t b_hash, const char *b_value, uint32_t b_len) {
68 if ((b_hash >> 16) != (a >> 48)) // high-order 16 bits are from the hash value
70 const string_t *a_key = GET_PTR(a);
72 return a_key->len == b_len && memcmp(a_key->val, b_value, b_len) == 0;
75 // Lookup <key> in <hti>.
77 // Return the entry that <key> is in, or if <key> isn't in <hti> return the entry that it would be
78 // in if it were inserted into <hti>. If there is no room for <key> in <hti> then return NULL, to
79 // indicate that the caller should look in <hti->next>.
81 // Record if the entry being returned is empty. Otherwise the caller will have to waste time with
82 // ht_key_equals() to confirm that it did not lose a race to fill an empty entry.
83 static volatile entry_t *hti_lookup (hash_table_i_t *hti, uint32_t key_hash, const char *key_val, uint32_t key_len, int *is_empty) {
84 TRACE("h0", "hti_lookup(key %p in hti %p)", key_val, hti);
87 // Probe one cache line at a time
88 int ndx = key_hash & MASK(hti->scale); // the first entry to search
89 for (int i = 0; i < hti->max_probe; ++i) {
91 // The start of the bucket is the first entry in the cache line.
92 volatile entry_t *bucket = hti->table + (ndx & ~(ENTRIES_PER_BUCKET-1));
94 // Start searching at the indexed entry. Then loop around to the begining of the cache line.
95 for (int j = 0; j < ENTRIES_PER_BUCKET; ++j) {
96 volatile entry_t *e = bucket + ((ndx + j) & (ENTRIES_PER_BUCKET-1));
98 uint64_t e_key = e->key;
99 if (e_key == DOES_NOT_EXIST) {
100 TRACE("h0", "hti_lookup: empty entry %p found", e, 0);
101 *is_empty = 1; // indicate an empty so the caller avoids an expensive ht_key_equals
105 if (ht_key_equals(e_key, key_hash, key_val, key_len)) {
106 TRACE("h0", "hti_lookup: entry %p found value %p", e, e->value);
107 TRACE("h0", "hti_lookup: entry key %p len %llu", GET_PTR(e_key)->val,
108 GET_PTR(e_key)->len);
113 ndx = get_next_ndx(ndx, key_hash, hti->scale);
116 // maximum number of probes exceeded
117 TRACE("h0", "hti_lookup: maximum number of probes exceeded returning 0x0", 0, 0);
121 // Allocate and initialize a hash_table_i_t with 2^<scale> entries.
122 static hash_table_i_t *hti_alloc (hash_table_t *parent, int scale) {
123 // Include enough slop to align the actual table on a cache line boundry
124 size_t n = sizeof(hash_table_i_t)
125 + sizeof(entry_t) * (1 << scale)
126 + (CACHE_LINE_SIZE - 1);
127 hash_table_i_t *hti = (hash_table_i_t *)calloc(n, 1);
129 // Align the table of hash entries on a cache line boundry.
130 hti->table = (entry_t *)(((uint64_t)hti + sizeof(hash_table_i_t) + (CACHE_LINE_SIZE-1))
131 & ~(CACHE_LINE_SIZE-1));
135 // When searching for a key probe a maximum of 1/4 of the buckets up to 1000 buckets.
136 hti->max_probe = ((1 << (hti->scale - 2)) / ENTRIES_PER_BUCKET) + 2;
137 if (hti->max_probe > MAX_BUCKETS_TO_PROBE) {
138 hti->max_probe = MAX_BUCKETS_TO_PROBE;
143 assert(hti->scale >= MIN_SCALE && hti->scale < 63); // size must be a power of 2
144 assert(sizeof(entry_t) * ENTRIES_PER_BUCKET % CACHE_LINE_SIZE == 0); // divisible into cache
145 assert((size_t)hti->table % CACHE_LINE_SIZE == 0); // cache aligned
150 // Called when <hti> runs out of room for new keys.
152 // Initiates a copy by creating a larger hash_table_i_t and installing it in <hti->next>.
153 static void hti_start_copy (hash_table_i_t *hti) {
154 TRACE("h0", "hti_start_copy(hti %p scale %llu)", hti, hti->scale);
156 // heuristics to determine the size of the new table
157 uint64_t count = ht_count(hti->ht);
158 unsigned int new_scale = hti->scale;
159 new_scale += (count > (1 << (new_scale - 2))); // double size if more than 1/4 full
160 new_scale += (count > (1 << (new_scale - 2))); // double size again if more than 1/2 full
162 // Allocate the new table and attempt to install it.
163 hash_table_i_t *next = hti_alloc(hti->ht, new_scale);
164 TRACE("h0", "hti_start_copy: new hti %p scale %llu", next->scale, next->scale);
165 hash_table_i_t *old_next = SYNC_CAS(&hti->next, NULL, next);
166 if (old_next != NULL) {
167 TRACE("h0", "hti_start_copy: lost race to install new hti; found %p", old_next, 0);
168 // Another thread beat us to it.
172 TRACE("h0", "hti_start_copy: new hti is %p", next, 0);
175 // Copy the key and value stored in <ht1_e> (which must be an entry in <ht1>) to <ht2>.
177 // Return 1 unless <ht1_e> is already copied (then return 0), so the caller can account for the total
178 // number of entries left to copy.
179 static int hti_copy_entry (hash_table_i_t *ht1, volatile entry_t *ht1_e, uint32_t key_hash,
180 hash_table_i_t *ht2) {
181 TRACE("h0", "hti_copy_entry(copy entry from %p to %p)", ht1, ht2);
185 assert(ht1_e >= ht1->table && ht1_e < ht1->table + (1 << ht1->scale));
186 assert(key_hash == 0 || (key_hash >> 16) == (ht1_e->key >> 48));
188 uint64_t ht1_e_value = ht1_e->value;
189 TRACE("h0", "hti_copy_entry: entry %p current value %p", ht1_e, ht1_e_value);
190 if (EXPECT_FALSE(ht1_e_value == COPIED_VALUE))
191 return FALSE; // already copied
193 // Kill empty entries.
194 if (EXPECT_FALSE(ht1_e_value == DOES_NOT_EXIST)) {
195 uint64_t ht1_e_value = SYNC_CAS(&ht1_e->value, DOES_NOT_EXIST, COPIED_VALUE);
196 if (ht1_e_value == DOES_NOT_EXIST) {
197 TRACE("h0", "hti_copy_entry: old entry killed", 0, 0);
200 if (ht1_e_value == COPIED_VALUE) {
201 TRACE("h0", "hti_copy_entry: lost race to kill empty entry in old hti", 0, 0);
202 return FALSE; // another thread beat us to it
204 TRACE("h0", "hti_copy_entry: lost race to kill empty entry in old hti; "
205 "the entry is now being used", 0, 0);
208 // Tag the value in the old entry to indicate a copy is in progress.
209 ht1_e_value = SYNC_FETCH_AND_OR(&ht1_e->value, TAG_VALUE(0));
210 TRACE("h0", "hti_copy_entry: tagged the value %p in old entry %p", ht1_e_value, ht1_e);
211 if (ht1_e_value == COPIED_VALUE)
212 return FALSE; // <value> was already copied by another thread.
214 // Deleted entries don't need to be installed into to the new table, but their keys do need to
216 assert(COPIED_VALUE == TAG_VALUE(TOMBSTONE));
217 if (ht1_e_value == TOMBSTONE) {
218 nbd_defer_free(GET_PTR(ht1_e->key));
222 // Install the key in the new table.
223 uint64_t key = ht1_e->key;
224 string_t *key_string = GET_PTR(key);
225 uint64_t value = STRIP_TAG(ht1_e_value);
226 TRACE("h0", "hti_copy_entry: key %p value %p", key, value);
228 // We use 0 to indicate that <key_hash> isn't initiallized. Occasionally the <key_hash> will
229 // really be 0 and we will waste time recomputing it. That is rare enough that it is OK.
231 key_hash = murmur32(key_string->val, key_string->len);
235 volatile entry_t *ht2_e = hti_lookup(ht2, key_hash, key_string->val, key_string->len, &is_empty);
237 // it is possible that there is not any room in the new table either
238 if (EXPECT_FALSE(ht2_e == NULL)) {
239 if (ht2->next == NULL) {
240 hti_start_copy(ht2); // initiate nested copy, if not already started
242 return hti_copy_entry(ht1, ht1_e, key_hash, ht2->next); // recursive tail-call
245 // a tagged entry returned from hti_lookup() means it is either empty or has a new key
247 uint64_t old_ht2_e_key = SYNC_CAS(&ht2_e->key, DOES_NOT_EXIST, key);
248 if (old_ht2_e_key != DOES_NOT_EXIST) {
249 TRACE("h0", "hti_copy_entry: lost race to CAS key %p into new entry; found %p",
251 return hti_copy_entry(ht1, ht1_e, key_hash, ht2); // recursive tail-call
254 assert(ht_key_equals(ht2_e->key, key_hash, key_string->val, key_string->len));
255 TRACE("h0", "hti_copy_entry: key %p installed in new hti %p", key_string->val, ht2);
257 // Copy the value to the entry in the new table.
258 uint64_t old_ht2_e_value = SYNC_CAS(&ht2_e->value, DOES_NOT_EXIST, value);
260 // If there is a nested copy in progress, we might have installed the key into a dead entry.
261 if (old_ht2_e_value == COPIED_VALUE)
262 return hti_copy_entry(ht1, ht1_e, key_hash, ht2->next); // recursive tail-call
264 // Mark the old entry as dead.
265 ht1_e->value = COPIED_VALUE;
267 // Update the count if we were the one that completed the copy.
268 if (old_ht2_e_value == DOES_NOT_EXIST) {
269 TRACE("h0", "hti_copy_entry: value %p installed in new hti %p", value, ht2);
270 SYNC_ADD(&ht1->count, -1);
271 SYNC_ADD(&ht2->count, 1);
275 TRACE("h0", "hti_copy_entry: lost race to CAS value %p in new hti; found %p",
276 value, old_ht2_e_value);
277 return FALSE; // another thread completed the copy
280 // Compare <expected> with the existing value associated with <key>. If the values match then
281 // replace the existing value with <new>. If <new> is TOMBSTONE, delete the value associated with
282 // the key by replacing it with a TOMBSTONE.
284 // Return the previous value associated with <key>, or DOES_NOT_EXIST if <key> is not in the table
285 // or associated with a TOMBSTONE. If a copy is in progress and <key> has been copied to the next
286 // table then return COPIED_VALUE.
288 // NOTE: the returned value matches <expected> iff the set succeeds
290 // Certain values of <expected> have special meaning. If <expected> is HT_EXPECT_EXISTS then any
291 // real value matches (i.e. not a TOMBSTONE or DOES_NOT_EXIST) as long as <key> is in the table. If
292 // <expected> is HT_EXPECT_WHATEVER then skip the test entirely.
294 static uint64_t hti_compare_and_set (hash_table_i_t *hti, uint32_t key_hash, const char *key_val,
295 uint32_t key_len, uint64_t expected, uint64_t new) {
296 TRACE("h0", "hti_compare_and_set: hti %p key %p", hti, key_val);
297 TRACE("h0", "hti_compare_and_set: new value %p expected old value %p", new, expected);
299 assert(new != DOES_NOT_EXIST && !IS_TAGGED(new));
303 volatile entry_t *e = hti_lookup(hti, key_hash, key_val, key_len, &is_empty);
305 // There is no room for <key>, grow the table and try again.
307 if (hti->next == NULL) {
313 // Install <key> in the table if it doesn't exist.
315 TRACE("h0", "hti_compare_and_set: entry %p is empty", e, 0);
316 if (expected != HT_EXPECT_WHATEVER && expected != HT_EXPECT_NOT_EXISTS)
317 return DOES_NOT_EXIST;
319 // No need to do anything, <key> is already deleted.
320 if (new == TOMBSTONE)
321 return DOES_NOT_EXIST;
324 string_t *key = nbd_malloc(sizeof(uint32_t) + key_len);
326 memcpy(key->val, key_val, key_len);
328 // Combine <key> pointer with bits from its hash, CAS it into the table.
329 uint64_t temp = ((uint64_t)(key_hash >> 16) << 48) | (uint64_t)key;
330 uint64_t e_key = SYNC_CAS(&e->key, DOES_NOT_EXIST, temp);
332 // Retry if another thread stole the entry out from under us.
333 if (e_key != DOES_NOT_EXIST) {
334 TRACE("h0", "hti_compare_and_set: lost race to install key %p in entry %p", key, e);
335 TRACE("h0", "hti_compare_and_set: found %p instead of NULL", GET_PTR(e_key), 0);
337 return hti_compare_and_set(hti, key_hash, key_val, key_len, expected, new); // tail-call
339 TRACE("h0", "hti_compare_and_set: installed key %p in entry %p", key, e);
342 // If the entry is in the middle of a copy, the copy must be completed first.
343 uint64_t e_value = e->value;
344 TRACE("h0", "hti_compare_and_set: value in entry %p is %p", e, e_value);
345 if (EXPECT_FALSE(IS_TAGGED(e_value))) {
346 int did_copy = hti_copy_entry(hti, e, key_hash, ((volatile hash_table_i_t *)hti)->next);
348 SYNC_ADD(&hti->num_entries_copied, 1);
353 // Fail if the old value is not consistent with the caller's expectation.
354 int old_existed = (e_value != TOMBSTONE && e_value != DOES_NOT_EXIST);
355 if (EXPECT_FALSE(expected != HT_EXPECT_WHATEVER && expected != e_value)) {
356 if (EXPECT_FALSE(expected != (old_existed ? HT_EXPECT_EXISTS : HT_EXPECT_NOT_EXISTS))) {
357 TRACE("h0", "hti_compare_and_set: value %p expected by caller not found; found value %p",
363 // CAS the value into the entry. Retry if it fails.
364 uint64_t v = SYNC_CAS(&e->value, e_value, new);
365 if (EXPECT_FALSE(v != e_value)) {
366 TRACE("h0", "hti_compare_and_set: value CAS failed; expected %p found %p", e_value, v);
367 return hti_compare_and_set(hti, key_hash, key_val, key_len, expected, new); // recursive tail-call
370 // The set succeeded. Adjust the value count.
371 if (old_existed && new == TOMBSTONE) {
372 SYNC_ADD(&hti->count, -1);
373 } else if (!old_existed && new != TOMBSTONE) {
374 SYNC_ADD(&hti->count, 1);
377 // Return the previous value.
378 TRACE("h0", "hti_compare_and_set: CAS succeeded; old value %p new value %p", e_value, new);
383 static uint64_t hti_get (hash_table_i_t *hti, uint32_t key_hash, const char *key_val, uint32_t key_len) {
387 volatile entry_t *e = hti_lookup(hti, key_hash, key_val, key_len, &is_empty);
389 // When hti_lookup() returns NULL it means we hit the reprobe limit while
390 // searching the table. In that case, if a copy is in progress the key
391 // might exist in the copy.
392 if (EXPECT_FALSE(e == NULL)) {
393 if (((volatile hash_table_i_t *)hti)->next != NULL)
394 return hti_get(hti->next, key_hash, key_val, key_len); // recursive tail-call
395 return DOES_NOT_EXIST;
399 return DOES_NOT_EXIST;
401 // If the entry is being copied, finish the copy and retry on the next table.
402 uint64_t e_value = e->value;
403 if (EXPECT_FALSE(IS_TAGGED(e_value))) {
404 if (EXPECT_FALSE(e_value != COPIED_VALUE)) {
405 int did_copy = hti_copy_entry(hti, e, key_hash, ((volatile hash_table_i_t *)hti)->next);
407 SYNC_ADD(&hti->num_entries_copied, 1);
410 return hti_get(((volatile hash_table_i_t *)hti)->next, key_hash, key_val, key_len); // tail-call
413 return (e_value == TOMBSTONE) ? DOES_NOT_EXIST : e_value;
417 uint64_t ht_get (hash_table_t *ht, const char *key_val, uint32_t key_len) {
418 return hti_get(*ht, murmur32(key_val, key_len), key_val, key_len);
422 uint64_t ht_compare_and_set (hash_table_t *ht, const char *key_val, uint32_t key_len,
423 uint64_t expected_val, uint64_t new_val) {
425 TRACE("h0", "ht_compare_and_set: key %p len %u", key_val, key_len);
426 TRACE("h0", "ht_compare_and_set: expected val %p new val %p", expected_val, new_val);
428 assert(!IS_TAGGED(new_val) && new_val != DOES_NOT_EXIST);
430 hash_table_i_t *hti = *ht;
432 // Help with an ongoing copy.
433 if (EXPECT_FALSE(hti->next != NULL)) {
439 TRACE("h0", "ht_compare_and_set: help copy. scan is %llu, size is %llu", x, 1<<hti->scale);
440 // Panic if we've been around the array twice and still haven't finished the copy.
441 int panic = (x >= (1 << (hti->scale + 1)));
443 limit = ENTRIES_PER_COPY_CHUNK;
445 // Reserve some entries for this thread to copy. There is a race condition here because the
446 // fetch and add isn't atomic, but that is ok.
447 hti->scan = x + ENTRIES_PER_COPY_CHUNK;
449 // <hti->scan> might be larger than the size of the table, if some thread stalls while
450 // copying. In that case we just wrap around to the begining and make another pass through
452 e = hti->table + (x & MASK(hti->scale));
454 TRACE("h0", "ht_compare_and_set: help copy panic", 0, 0);
455 // scan the whole table
456 limit = (1 << hti->scale);
461 for (int i = 0; i < limit; ++i) {
462 num_copied += hti_copy_entry(hti, e++, 0, hti->next);
463 assert(e <= hti->table + (1 << hti->scale));
465 if (num_copied != 0) {
466 SYNC_ADD(&hti->num_entries_copied, num_copied);
469 // Dispose of fully copied tables.
470 if (hti->num_entries_copied == (1 << hti->scale) || panic) {
472 if (SYNC_CAS(ht, hti, hti->next) == hti) {
479 uint32_t key_hash = murmur32(key_val, key_len);
480 while ((old_val = hti_compare_and_set(hti, key_hash, key_val, key_len, expected_val, new_val))
486 return old_val == TOMBSTONE ? DOES_NOT_EXIST : old_val;
489 // Remove the value in <ht> associated with <key_val>. Returns the value removed, or
490 // DOES_NOT_EXIST if there was no value for that key.
491 uint64_t ht_remove (hash_table_t *ht, const char *key_val, uint32_t key_len) {
492 hash_table_i_t *hti = *ht;
494 uint32_t key_hash = murmur32(key_val, key_len);
496 val = hti_compare_and_set(hti, key_hash, key_val, key_len, HT_EXPECT_WHATEVER, TOMBSTONE);
497 if (val != COPIED_VALUE)
498 return val == TOMBSTONE ? DOES_NOT_EXIST : val;
505 // Returns the number of key-values pairs in <ht>
506 uint64_t ht_count (hash_table_t *ht) {
507 hash_table_i_t *hti = *ht;
516 // Allocate and initialize a new hash table.
517 hash_table_t *ht_alloc (void) {
518 hash_table_t *ht = nbd_malloc(sizeof(hash_table_t));
519 *ht = (hash_table_i_t *)hti_alloc(ht, MIN_SCALE);
523 // Free <ht> and its internal structures.
524 void ht_free (hash_table_t *ht) {
525 hash_table_i_t *hti = *ht;
527 for (uint32_t i = 0; i < (1 << hti->scale); ++i) {
528 assert(hti->table[i].value == COPIED_VALUE || !IS_TAGGED(hti->table[i].value));
529 if (hti->table[i].key != DOES_NOT_EXIST) {
530 nbd_free(GET_PTR(hti->table[i].key));
533 hash_table_i_t *next = hti->next;