--- /dev/null
+/*
+ Copyright (c) 2012 Cisco and/or its affiliates.
+
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+*/
+
+#include <vppinfra/slist.h>
+
+/*
+ * skip-list implementation
+ *
+ * Good news / bad news. As balanced binary tree schemes go,
+ * this one seems pretty fast and is reasonably simple. There's a very
+ * limited amount that can be done to mitigate sdram read latency.
+ *
+ * Each active clib_slist_elt_t is on from 1 to N lists. Each active element
+ * is always on the "level-0" list. Since most elements are *only* on
+ * level 0, we keep the level 0 (and level 1) in the element. For those
+ * elements on more than two lists, we switch to a vector. Hence, the
+ * "n" union in slib_slist_elt_t.
+ *
+ * The low-order bit of elt->n.next0[0] is 1 for inlined next indices,
+ * 0 for vector indices (since the allocator always aligns to at least
+ * a 4-byte boundary). We can only represent 2e9 items, but since the
+ * practical performance limit is O(1e7), it doesn't matter.
+ *
+ * We create a "head" element which (by construction) is always
+ * lexically lighter than any other element. This makes a large number
+ * of irritating special cases go away.
+ *
+ * User code is in charge of comparing a supplied key with
+ * the key component of a user pool element. The user tells this code
+ * to add or delete (opaque key, 32-bit integer) pairs to the skip-list.
+ *
+ * The algorithm adds new elements to one or more lists.
+ * For levels greater than zero, the probability of a new element landing on
+ * a list is branching_factor**N. Branching_factor = 0.2 seems to work
+ * OK, yielding about 50 compares per search at O(1e7) items.
+ */
+
+clib_error_t *
+clib_slist_init (clib_slist_t * sp, f64 branching_factor,
+ clib_slist_key_compare_function_t compare,
+ format_function_t format_user_element)
+{
+ clib_slist_elt_t *head;
+ memset (sp, 0, sizeof (sp[0]));
+ sp->branching_factor = branching_factor;
+ sp->format_user_element = format_user_element;
+ sp->compare = compare;
+ sp->seed = 0xdeaddabe;
+ pool_get (sp->elts, head);
+ vec_add1 (head->n.nexts, (u32) ~ 0);
+ head->user_pool_index = (u32) ~ 0;
+ vec_validate (sp->path, 1);
+ vec_validate (sp->occupancy, 0);
+
+ return 0;
+}
+
+/*
+ * slist_search_internal
+ */
+static inline clib_slist_search_result_t
+slist_search_internal (clib_slist_t * sp, void *key, int need_full_path)
+{
+ int level, comp_result;
+ clib_slist_elt_t *search_elt, *head_elt;
+
+ sp->ncompares = 0;
+ /*
+ * index 0 is the magic listhead element which is
+ * lexically lighter than / to the left of every element
+ */
+ search_elt = head_elt = pool_elt_at_index (sp->elts, 0);
+
+ /*
+ * Initial negotiating position, only the head_elt is
+ * lighter than the supplied key
+ */
+ memset (sp->path, 0, vec_len (head_elt->n.nexts) * sizeof (u32));
+
+ /* Walk the fastest lane first */
+ level = vec_len (head_elt->n.nexts) - 1;
+ _vec_len (sp->path) = level + 1;
+
+ while (1)
+ {
+ u32 next_index_this_level;
+ clib_slist_elt_t *prefetch_elt;
+
+ /*
+ * Prefetching the next element at this level makes a measurable
+ * difference, but doesn't fix the dependent read stall problem
+ */
+ prefetch_elt = sp->elts +
+ clib_slist_get_next_at_level (search_elt, level);
+
+ CLIB_PREFETCH (prefetch_elt, CLIB_CACHE_LINE_BYTES, READ);
+
+ /* Compare the key with the current element */
+ comp_result = (search_elt == head_elt) ? 1 :
+ sp->compare (key, search_elt->user_pool_index);
+
+ sp->ncompares++;
+ /* key "lighter" than this element */
+ if (comp_result < 0)
+ {
+ /*
+ * Back up to previous item on this list
+ * and search the next finer-grained list
+ * starting there.
+ */
+ search_elt = pool_elt_at_index (sp->elts, sp->path[level]);
+ next_list:
+ if (level > 0)
+ {
+ level--;
+ continue;
+ }
+ else
+ {
+ return CLIB_SLIST_NO_MATCH;
+ }
+ }
+ /* Match */
+ if (comp_result == 0)
+ {
+ /*
+ * If we're trying to delete an element, we need to
+ * track down all of the elements which point at it.
+ * Otherwise, don't bother with it
+ */
+ if (need_full_path && level > 0)
+ {
+ search_elt = pool_elt_at_index (sp->elts, sp->path[level]);
+ level--;
+ continue;
+ }
+ level = vec_len (head_elt->n.nexts);
+ sp->path[level] = search_elt - sp->elts;
+ _vec_len (sp->path) = level + 1;
+ return CLIB_SLIST_MATCH;
+ }
+ /*
+ * comp_result positive, key is to the right of
+ * this element
+ */
+ sp->path[level] = search_elt - sp->elts;
+
+ /* Out of list at this level? */
+ next_index_this_level =
+ clib_slist_get_next_at_level (search_elt, level);
+ if (next_index_this_level == (u32) ~ 0)
+ goto next_list;
+
+ /* No, try the next element */
+ search_elt = pool_elt_at_index (sp->elts, next_index_this_level);
+ }
+ return 0; /* notreached */
+}
+
+u32
+clib_slist_search (clib_slist_t * sp, void *key, u32 * ncompares)
+{
+ clib_slist_search_result_t rv;
+
+ rv = slist_search_internal (sp, key, 0 /* dont need full path */ );
+ if (rv == CLIB_SLIST_MATCH)
+ {
+ clib_slist_elt_t *elt;
+ elt = pool_elt_at_index (sp->elts, sp->path[vec_len (sp->path) - 1]);
+ if (ncompares)
+ *ncompares = sp->ncompares;
+ return elt->user_pool_index;
+ }
+ return (u32) ~ 0;
+}
+
+void
+clib_slist_add (clib_slist_t * sp, void *key, u32 user_pool_index)
+{
+ clib_slist_elt_t *new_elt;
+ clib_slist_search_result_t search_result;
+ int level;
+
+ search_result = slist_search_internal (sp, key,
+ 0 /* don't need full path */ );
+
+ /* Special case: key exists, just replace user_pool_index */
+ if (PREDICT_FALSE (search_result == CLIB_SLIST_MATCH))
+ {
+ clib_slist_elt_t *elt;
+ elt = pool_elt_at_index (sp->elts, sp->path[0]);
+ elt->user_pool_index = user_pool_index;
+ return;
+ }
+
+ pool_get (sp->elts, new_elt);
+ new_elt->n.nexts = 0;
+ new_elt->user_pool_index = user_pool_index;
+
+ /* sp->path lists elements to the left of key, by level */
+ for (level = 0; level < vec_len (sp->path); level++)
+ {
+ clib_slist_elt_t *prev_elt_this_level;
+ u32 prev_elt_next_index_this_level;
+
+ /* Add to list at the current level */
+ prev_elt_this_level = pool_elt_at_index (sp->elts, sp->path[level]);
+ prev_elt_next_index_this_level = clib_slist_get_next_at_level
+ (prev_elt_this_level, level);
+
+ clib_slist_set_next_at_level (new_elt, prev_elt_next_index_this_level,
+ level);
+
+ clib_slist_set_next_at_level (prev_elt_this_level, new_elt - sp->elts,
+ level);
+ sp->occupancy[level]++;
+
+ /* Randomly add to the next-higher level */
+ if (random_f64 (&sp->seed) > sp->branching_factor)
+ break;
+ }
+ {
+ /* Time to add a new ply? */
+ clib_slist_elt_t *head_elt = pool_elt_at_index (sp->elts, 0);
+ int top_level = vec_len (head_elt->n.nexts) - 1;
+ if (((f64) sp->occupancy[top_level]) * sp->branching_factor > 1.0)
+ {
+ vec_add1 (sp->occupancy, 0);
+ vec_add1 (head_elt->n.nexts, (u32) ~ 0);
+ /* full match case returns n+1 items */
+ vec_validate (sp->path, vec_len (head_elt->n.nexts));
+ }
+ }
+}
+
+clib_slist_search_result_t
+clib_slist_del (clib_slist_t * sp, void *key)
+{
+ clib_slist_search_result_t search_result;
+ clib_slist_elt_t *del_elt;
+ int level;
+
+ search_result = slist_search_internal (sp, key, 1 /* need full path */ );
+
+ if (PREDICT_FALSE (search_result == CLIB_SLIST_NO_MATCH))
+ return search_result;
+
+ del_elt = pool_elt_at_index (sp->elts, sp->path[vec_len (sp->path) - 1]);
+ ASSERT (vec_len (sp->path) > 1);
+
+ for (level = 0; level < vec_len (sp->path) - 1; level++)
+ {
+ clib_slist_elt_t *path_elt;
+ u32 path_elt_next_index;
+
+ path_elt = pool_elt_at_index (sp->elts, sp->path[level]);
+ path_elt_next_index = clib_slist_get_next_at_level (path_elt, level);
+
+ /* Splice the item out of the list if it's adjacent to the victim */
+ if (path_elt_next_index == del_elt - sp->elts)
+ {
+ sp->occupancy[level]--;
+ path_elt_next_index = clib_slist_get_next_at_level (del_elt, level);
+ clib_slist_set_next_at_level (path_elt, path_elt_next_index, level);
+ }
+ }
+
+ /* If this element is on more than two lists it has a vector of nexts */
+ if (!(del_elt->n.next0[0] & 1))
+ vec_free (del_elt->n.nexts);
+ pool_put (sp->elts, del_elt);
+ return CLIB_SLIST_MATCH;
+}
+
+u8 *
+format_slist (u8 * s, va_list * args)
+{
+ clib_slist_t *sl = va_arg (*args, clib_slist_t *);
+ int verbose = va_arg (*args, int);
+ int i;
+ clib_slist_elt_t *head_elt, *elt;
+
+ s = format (s, "slist 0x%x, %u items, branching_factor %.2f\n", sl,
+ sl->occupancy ? sl->occupancy[0] : 0, sl->branching_factor);
+
+ if (pool_elts (sl->elts) == 0)
+ return s;
+
+ head_elt = pool_elt_at_index (sl->elts, 0);
+
+ for (i = 0; i < vec_len (head_elt->n.nexts); i++)
+ {
+ s = format (s, "level %d: %d elts\n", i,
+ sl->occupancy ? sl->occupancy[i] : 0);
+
+ if (verbose && head_elt->n.nexts[i] != (u32) ~ 0)
+ {
+ elt = pool_elt_at_index (sl->elts, head_elt->n.nexts[i]);
+ while (elt)
+ {
+ u32 next_index;
+ s = format (s, "%U(%d) ", sl->format_user_element,
+ elt->user_pool_index, elt - sl->elts);
+ next_index = clib_slist_get_next_at_level (elt, i);
+ ASSERT (next_index != 0x7fffffff);
+ if (next_index == (u32) ~ 0)
+ break;
+ else
+ elt = pool_elt_at_index (sl->elts, next_index);
+ }
+ }
+ s = format (s, "\n");
+ }
+ return s;
+}
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
Code Review / vpp.git / blobdiff