rcuja: implement 2d distribution
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Thu, 30 May 2013 01:54:09 +0000 (21:54 -0400)
committerMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Thu, 30 May 2013 01:54:09 +0000 (21:54 -0400)
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
rcuja/rcuja-internal.h
rcuja/rcuja.c

index eaef469b031f45387878ca63f9fbcac3db4f721c..1414b8f43ea46fa32822a1ab252789091e960462 100644 (file)
@@ -113,6 +113,16 @@ struct cds_ja_inode_flag *ja_node_flag_pool_1d(struct cds_ja_inode *node,
        return (struct cds_ja_inode_flag *) (((unsigned long) node) | (bitsel << JA_TYPE_BITS) | type);
 }
 
+static inline
+struct cds_ja_inode_flag *ja_node_flag_pool_2d(struct cds_ja_inode *node,
+               unsigned long type, unsigned int bitsel[2])
+{
+       assert(type < (1UL << JA_TYPE_BITS));
+       assert(bitsel[0] < JA_BITS_PER_BYTE);
+       assert(bitsel[1] < JA_BITS_PER_BYTE);
+       return (struct cds_ja_inode_flag *) (((unsigned long) node) | (bitsel[0] << (JA_TYPE_BITS + JA_LOG2_BITS_PER_BYTE)) | (bitsel[1] << JA_TYPE_BITS) | type);
+}
+
 static inline
 unsigned long ja_node_pool_1d_bitsel(struct cds_ja_inode_flag *node)
 {
index fb1dccb7f093e167034bd6042846c4c8f53ce2ec..aa6350fe95d73d6ce98d45d22036bf52cfcdd551 100644 (file)
@@ -239,11 +239,14 @@ struct cds_ja_inode {
 };
 
 enum ja_recompact {
-       JA_RECOMPACT,
-       JA_RECOMPACT_ADD,
+       JA_RECOMPACT_ADD_SAME,
+       JA_RECOMPACT_ADD_NEXT,
        JA_RECOMPACT_DEL,
 };
 
+static
+unsigned long node_fallback_count_distribution[JA_ENTRY_PER_NODE];
+
 static
 struct cds_ja_inode *_ja_node_mask_ptr(struct cds_ja_inode_flag *node)
 {
@@ -410,18 +413,22 @@ struct cds_ja_inode_flag *ja_pool_node_get_nth(const struct cds_ja_type *type,
 
                bitsel = ja_node_pool_1d_bitsel(node_flag);
                assert(bitsel < CHAR_BIT);
-               index = ((unsigned long) n >> bitsel) & type->nr_pool_order;
+               index = ((unsigned long) n >> bitsel) & 0x1;
                linear = (struct cds_ja_inode *) &node->u.data[index << type->pool_size_order];
                break;
        }
        case 2:
        {
-               /*
-                * TODO: currently, we select the pool by highest bits. We
-                * should support various encodings.
-                */
-               linear = (struct cds_ja_inode *)
-                       &node->u.data[((unsigned long) n >> (CHAR_BIT - type->nr_pool_order)) << type->pool_size_order];
+               unsigned long bitsel[2], index[2], rindex;
+
+               ja_node_pool_2d_bitsel(node_flag, bitsel);
+               assert(bitsel[0] < CHAR_BIT);
+               assert(bitsel[1] < CHAR_BIT);
+               index[0] = ((unsigned long) n >> bitsel[0]) & 0x1;
+               index[0] <<= 1;
+               index[1] = ((unsigned long) n >> bitsel[1]) & 0x1;
+               rindex = index[0] | index[1];
+               linear = (struct cds_ja_inode *) &node->u.data[rindex << type->pool_size_order];
                break;
        }
        default:
@@ -591,18 +598,22 @@ int ja_pool_node_set_nth(const struct cds_ja_type *type,
 
                bitsel = ja_node_pool_1d_bitsel(node_flag);
                assert(bitsel < CHAR_BIT);
-               index = ((unsigned long) n >> bitsel) & type->nr_pool_order;
+               index = ((unsigned long) n >> bitsel) & 0x1;
                linear = (struct cds_ja_inode *) &node->u.data[index << type->pool_size_order];
                break;
        }
        case 2:
        {
-               /*
-                * TODO: currently, we select the pool by highest bits. We
-                * should support various encodings.
-                */
-               linear = (struct cds_ja_inode *)
-                       &node->u.data[((unsigned long) n >> (CHAR_BIT - type->nr_pool_order)) << type->pool_size_order];
+               unsigned long bitsel[2], index[2], rindex;
+
+               ja_node_pool_2d_bitsel(node_flag, bitsel);
+               assert(bitsel[0] < CHAR_BIT);
+               assert(bitsel[1] < CHAR_BIT);
+               index[0] = ((unsigned long) n >> bitsel[0]) & 0x1;
+               index[0] <<= 1;
+               index[1] = ((unsigned long) n >> bitsel[1]) & 0x1;
+               rindex = index[0] | index[1];
+               linear = (struct cds_ja_inode *) &node->u.data[rindex << type->pool_size_order];
                break;
        }
        default:
@@ -676,18 +687,19 @@ int ja_linear_node_clear_ptr(const struct cds_ja_type *type,
        assert(type->type_class == RCU_JA_LINEAR || type->type_class == RCU_JA_POOL);
 
        nr_child_ptr = &node->u.data[0];
-       dbg_printf("linear clear ptr: nr_child_ptr %p\n", nr_child_ptr);
        nr_child = *nr_child_ptr;
        assert(nr_child <= type->max_linear_child);
 
        if (shadow_node->fallback_removal_count) {
                shadow_node->fallback_removal_count--;
        } else {
-               if (shadow_node->nr_child <= type->min_child) {
+               if (type->type_class == RCU_JA_LINEAR
+                               && shadow_node->nr_child <= type->min_child) {
                        /* We need to try recompacting the node */
                        return -EFBIG;
                }
        }
+       dbg_printf("linear clear ptr: nr_child_ptr %p\n", nr_child_ptr);
        assert(*node_flag_ptr != NULL);
        rcu_assign_pointer(*node_flag_ptr, NULL);
        /*
@@ -702,13 +714,13 @@ int ja_linear_node_clear_ptr(const struct cds_ja_type *type,
                (unsigned int) CMM_LOAD_SHARED(*nr_child_ptr),
                (unsigned int) shadow_node->nr_child,
                node, shadow_node);
-
        return 0;
 }
 
 static
 int ja_pool_node_clear_ptr(const struct cds_ja_type *type,
                struct cds_ja_inode *node,
+               struct cds_ja_inode_flag *node_flag,
                struct cds_ja_shadow_node *shadow_node,
                struct cds_ja_inode_flag **node_flag_ptr,
                uint8_t n)
@@ -716,8 +728,45 @@ int ja_pool_node_clear_ptr(const struct cds_ja_type *type,
        struct cds_ja_inode *linear;
 
        assert(type->type_class == RCU_JA_POOL);
-       linear = (struct cds_ja_inode *)
-               &node->u.data[((unsigned long) n >> (CHAR_BIT - type->nr_pool_order)) << type->pool_size_order];
+
+       if (shadow_node->fallback_removal_count) {
+               shadow_node->fallback_removal_count--;
+       } else {
+               /* We should try recompacting the node */
+               if (shadow_node->nr_child <= type->min_child)
+                       return -EFBIG;
+       }
+
+       switch (type->nr_pool_order) {
+       case 1:
+       {
+               unsigned long bitsel, index;
+
+               bitsel = ja_node_pool_1d_bitsel(node_flag);
+               assert(bitsel < CHAR_BIT);
+               index = ((unsigned long) n >> bitsel) & type->nr_pool_order;
+               linear = (struct cds_ja_inode *) &node->u.data[index << type->pool_size_order];
+               break;
+       }
+       case 2:
+       {
+               unsigned long bitsel[2], index[2], rindex;
+
+               ja_node_pool_2d_bitsel(node_flag, bitsel);
+               assert(bitsel[0] < CHAR_BIT);
+               assert(bitsel[1] < CHAR_BIT);
+               index[0] = ((unsigned long) n >> bitsel[0]) & 0x1;
+               index[0] <<= 1;
+               index[1] = ((unsigned long) n >> bitsel[1]) & 0x1;
+               rindex = index[0] | index[1];
+               linear = (struct cds_ja_inode *) &node->u.data[rindex << type->pool_size_order];
+               break;
+       }
+       default:
+               linear = NULL;
+               assert(0);
+       }
+
        return ja_linear_node_clear_ptr(type, linear, shadow_node, node_flag_ptr);
 }
 
@@ -728,6 +777,14 @@ int ja_pigeon_node_clear_ptr(const struct cds_ja_type *type,
                struct cds_ja_inode_flag **node_flag_ptr)
 {
        assert(type->type_class == RCU_JA_PIGEON);
+
+       if (shadow_node->fallback_removal_count) {
+               shadow_node->fallback_removal_count--;
+       } else {
+               /* We should try recompacting the node */
+               if (shadow_node->nr_child <= type->min_child)
+                       return -EFBIG;
+       }
        dbg_printf("ja_pigeon_node_clear_ptr: clearing ptr: %p\n", *node_flag_ptr);
        rcu_assign_pointer(*node_flag_ptr, NULL);
        shadow_node->nr_child--;
@@ -741,6 +798,7 @@ int ja_pigeon_node_clear_ptr(const struct cds_ja_type *type,
 static
 int _ja_node_clear_ptr(const struct cds_ja_type *type,
                struct cds_ja_inode *node,
+               struct cds_ja_inode_flag *node_flag,
                struct cds_ja_shadow_node *shadow_node,
                struct cds_ja_inode_flag **node_flag_ptr,
                uint8_t n)
@@ -749,7 +807,7 @@ int _ja_node_clear_ptr(const struct cds_ja_type *type,
        case RCU_JA_LINEAR:
                return ja_linear_node_clear_ptr(type, node, shadow_node, node_flag_ptr);
        case RCU_JA_POOL:
-               return ja_pool_node_clear_ptr(type, node, shadow_node, node_flag_ptr, n);
+               return ja_pool_node_clear_ptr(type, node, node_flag, shadow_node, node_flag_ptr, n);
        case RCU_JA_PIGEON:
                return ja_pigeon_node_clear_ptr(type, node, shadow_node, node_flag_ptr);
        case RCU_JA_NULL:
@@ -863,14 +921,14 @@ unsigned int ja_node_sum_distribution_1d(enum ja_recompact mode,
                break;
        }
        case RCU_JA_NULL:
-               assert(mode == JA_RECOMPACT_ADD);
+               assert(mode == JA_RECOMPACT_ADD_NEXT);
                break;
        default:
                assert(0);
                break;
        }
 
-       if (mode == JA_RECOMPACT_ADD) {
+       if (mode == JA_RECOMPACT_ADD_NEXT || mode == JA_RECOMPACT_ADD_SAME) {
                for (bit_i = 0; bit_i < JA_BITS_PER_BYTE; bit_i++) {
                        if (n & (1U << bit_i))
                                nr_one[bit_i]++;
@@ -897,6 +955,218 @@ unsigned int ja_node_sum_distribution_1d(enum ja_recompact mode,
        return bitsel;
 }
 
+/*
+ * Calculate bit distribution in two dimensions. Returns the two bits
+ * (each 0 to 7) that splits the distribution in four sub-distributions
+ * containing as much elements one compared to the other.
+ */
+static
+void ja_node_sum_distribution_2d(enum ja_recompact mode,
+               struct cds_ja *ja,
+               unsigned int type_index,
+               const struct cds_ja_type *type,
+               struct cds_ja_inode *node,
+               struct cds_ja_shadow_node *shadow_node,
+               uint8_t n,
+               struct cds_ja_inode_flag *child_node_flag,
+               struct cds_ja_inode_flag **nullify_node_flag_ptr,
+               unsigned int *_bitsel)
+{
+       uint8_t nr_2d_11[JA_BITS_PER_BYTE][JA_BITS_PER_BYTE],
+               nr_2d_10[JA_BITS_PER_BYTE][JA_BITS_PER_BYTE],
+               nr_2d_01[JA_BITS_PER_BYTE][JA_BITS_PER_BYTE],
+               nr_2d_00[JA_BITS_PER_BYTE][JA_BITS_PER_BYTE];
+       unsigned int bitsel[2] = { 0, 1 };
+       unsigned int bit_i, bit_j, overall_best_distance = UINT_MAX;
+       unsigned int distrib_nr_child = 0;
+
+       memset(nr_2d_11, 0, sizeof(nr_2d_11));
+       memset(nr_2d_10, 0, sizeof(nr_2d_10));
+
+       switch (type->type_class) {
+       case RCU_JA_LINEAR:
+       {
+               uint8_t nr_child =
+                       ja_linear_node_get_nr_child(type, node);
+               unsigned int i;
+
+               for (i = 0; i < nr_child; i++) {
+                       struct cds_ja_inode_flag *iter;
+                       uint8_t v;
+
+                       ja_linear_node_get_ith_pos(type, node, i, &v, &iter);
+                       if (!iter)
+                               continue;
+                       if (mode == JA_RECOMPACT_DEL && *nullify_node_flag_ptr == iter)
+                               continue;
+                       for (bit_i = 0; bit_i < JA_BITS_PER_BYTE; bit_i++) {
+                               for (bit_j = 0; bit_j < bit_i; bit_j++) {
+                                       if ((v & (1U << bit_i)) && (v & (1U << bit_j))) {
+                                               nr_2d_11[bit_i][bit_j]++;
+                                       }
+                                       if ((v & (1U << bit_i)) && !(v & (1U << bit_j))) {
+                                               nr_2d_10[bit_i][bit_j]++;
+                                       }
+                                       if (!(v & (1U << bit_i)) && (v & (1U << bit_j))) {
+                                               nr_2d_01[bit_i][bit_j]++;
+                                       }
+                                       if (!(v & (1U << bit_i)) && !(v & (1U << bit_j))) {
+                                               nr_2d_00[bit_i][bit_j]++;
+                                       }
+                               }
+                       }
+                       distrib_nr_child++;
+               }
+               break;
+       }
+       case RCU_JA_POOL:
+       {
+               unsigned int pool_nr;
+
+               for (pool_nr = 0; pool_nr < (1U << type->nr_pool_order); pool_nr++) {
+                       struct cds_ja_inode *pool =
+                               ja_pool_node_get_ith_pool(type,
+                                       node, pool_nr);
+                       uint8_t nr_child =
+                               ja_linear_node_get_nr_child(type, pool);
+                       unsigned int j;
+
+                       for (j = 0; j < nr_child; j++) {
+                               struct cds_ja_inode_flag *iter;
+                               uint8_t v;
+
+                               ja_linear_node_get_ith_pos(type, pool,
+                                               j, &v, &iter);
+                               if (!iter)
+                                       continue;
+                               if (mode == JA_RECOMPACT_DEL && *nullify_node_flag_ptr == iter)
+                                       continue;
+                               for (bit_i = 0; bit_i < JA_BITS_PER_BYTE; bit_i++) {
+                                       for (bit_j = 0; bit_j < bit_i; bit_j++) {
+                                               if ((v & (1U << bit_i)) && (v & (1U << bit_j))) {
+                                                       nr_2d_11[bit_i][bit_j]++;
+                                               }
+                                               if ((v & (1U << bit_i)) && !(v & (1U << bit_j))) {
+                                                       nr_2d_10[bit_i][bit_j]++;
+                                               }
+                                               if (!(v & (1U << bit_i)) && (v & (1U << bit_j))) {
+                                                       nr_2d_01[bit_i][bit_j]++;
+                                               }
+                                               if (!(v & (1U << bit_i)) && !(v & (1U << bit_j))) {
+                                                       nr_2d_00[bit_i][bit_j]++;
+                                               }
+                                       }
+                               }
+                               distrib_nr_child++;
+                       }
+               }
+               break;
+       }
+       case RCU_JA_PIGEON:
+       {
+               uint8_t nr_child;
+               unsigned int i;
+
+               assert(mode == JA_RECOMPACT_DEL);
+               nr_child = shadow_node->nr_child;
+               for (i = 0; i < nr_child; i++) {
+                       struct cds_ja_inode_flag *iter;
+
+                       iter = ja_pigeon_node_get_ith_pos(type, node, i);
+                       if (!iter)
+                               continue;
+                       if (mode == JA_RECOMPACT_DEL && *nullify_node_flag_ptr == iter)
+                               continue;
+                       for (bit_i = 0; bit_i < JA_BITS_PER_BYTE; bit_i++) {
+                               for (bit_j = 0; bit_j < bit_i; bit_j++) {
+                                       if ((i & (1U << bit_i)) && (i & (1U << bit_j))) {
+                                               nr_2d_11[bit_i][bit_j]++;
+                                       }
+                                       if ((i & (1U << bit_i)) && !(i & (1U << bit_j))) {
+                                               nr_2d_10[bit_i][bit_j]++;
+                                       }
+                                       if (!(i & (1U << bit_i)) && (i & (1U << bit_j))) {
+                                               nr_2d_01[bit_i][bit_j]++;
+                                       }
+                                       if (!(i & (1U << bit_i)) && !(i & (1U << bit_j))) {
+                                               nr_2d_00[bit_i][bit_j]++;
+                                       }
+                               }
+                       }
+                       distrib_nr_child++;
+               }
+               break;
+       }
+       case RCU_JA_NULL:
+               assert(mode == JA_RECOMPACT_ADD_NEXT);
+               break;
+       default:
+               assert(0);
+               break;
+       }
+
+       if (mode == JA_RECOMPACT_ADD_NEXT || mode == JA_RECOMPACT_ADD_SAME) {
+               for (bit_i = 0; bit_i < JA_BITS_PER_BYTE; bit_i++) {
+                       for (bit_j = 0; bit_j < bit_i; bit_j++) {
+                               if ((n & (1U << bit_i)) && (n & (1U << bit_j))) {
+                                       nr_2d_11[bit_i][bit_j]++;
+                               }
+                               if ((n & (1U << bit_i)) && !(n & (1U << bit_j))) {
+                                       nr_2d_10[bit_i][bit_j]++;
+                               }
+                               if (!(n & (1U << bit_i)) && (n & (1U << bit_j))) {
+                                       nr_2d_01[bit_i][bit_j]++;
+                               }
+                               if (!(n & (1U << bit_i)) && !(n & (1U << bit_j))) {
+                                       nr_2d_00[bit_i][bit_j]++;
+                               }
+                       }
+               }
+               distrib_nr_child++;
+       }
+
+       /*
+        * The best bit selector is that for which the number of nodes
+        * in each sub-class is closest to one-fourth of the number of
+        * children in the distribution. We calculate the distance using
+        * 4 times the size of the sub-distribution to eliminate
+        * truncation error.
+        */
+       for (bit_i = 0; bit_i < JA_BITS_PER_BYTE; bit_i++) {
+               for (bit_j = 0; bit_j < bit_i; bit_j++) {
+                       unsigned int distance_to_best[4];
+
+                       distance_to_best[0] = nr_2d_11[bit_i][bit_j];
+                       distance_to_best[1] = nr_2d_10[bit_i][bit_j];
+                       distance_to_best[2] = nr_2d_01[bit_i][bit_j];
+                       distance_to_best[3] = nr_2d_00[bit_i][bit_j];
+
+                       /* Consider worse distance to best */
+                       if (distance_to_best[1] > distance_to_best[0])
+                               distance_to_best[0] = distance_to_best[1];
+                       if (distance_to_best[2] > distance_to_best[0])
+                               distance_to_best[0] = distance_to_best[2];
+                       if (distance_to_best[3] > distance_to_best[0])
+                               distance_to_best[0] = distance_to_best[3];
+                       /*
+                        * If our worse distance is better than overall,
+                        * we become new best candidate.
+                        */
+                       if (distance_to_best[0] < overall_best_distance) {
+                               overall_best_distance = distance_to_best[0];
+                               bitsel[0] = bit_i;
+                               bitsel[1] = bit_j;
+                       }
+               }
+       }
+
+       dbg_printf("2 dimensions pool bit selection: (%u,%u)\n", bitsel[0], bitsel[1]);
+
+       /* Return our bit selection */
+       _bitsel[0] = bitsel[0];
+       _bitsel[1] = bitsel[1];
+}
+
 /*
  * ja_node_recompact_add: recompact a node, adding a new child.
  * Return 0 on success, -EAGAIN if need to retry, or other negative
@@ -924,21 +1194,62 @@ int ja_node_recompact(enum ja_recompact mode,
        old_node_flag = *old_node_flag_ptr;
 
        switch (mode) {
-       case JA_RECOMPACT:
-               new_type_index = old_type_index;
+       case JA_RECOMPACT_ADD_SAME:
+               if (old_type->type_class == RCU_JA_POOL) {
+                       /*
+                        * For pool type, try redistributing
+                        * into a different distribution of same
+                        * size if we have not reached limits.
+                        */
+                       if (shadow_node->nr_child + 1 > old_type->max_child) {
+                               new_type_index = old_type_index + 1;
+                       } else if (shadow_node->nr_child + 1 < old_type->min_child) {
+                               new_type_index = old_type_index - 1;
+                       } else {
+                               new_type_index = old_type_index;
+                       }
+               } else {
+                       new_type_index = old_type_index;
+               }
                break;
-       case JA_RECOMPACT_ADD:
+       case JA_RECOMPACT_ADD_NEXT:
                if (!shadow_node || old_type_index == NODE_INDEX_NULL) {
                        new_type_index = 0;
                } else {
-                       new_type_index = old_type_index + 1;
+                       if (old_type->type_class == RCU_JA_POOL) {
+                               /*
+                                * For pool type, try redistributing
+                                * into a different distribution of same
+                                * size if we have not reached limits.
+                                */
+                               if (shadow_node->nr_child + 1 > old_type->max_child) {
+                                       new_type_index = old_type_index + 1;
+                               } else {
+                                       new_type_index = old_type_index;
+                               }
+                       } else {
+                               new_type_index = old_type_index + 1;
+                       }
                }
                break;
        case JA_RECOMPACT_DEL:
                if (old_type_index == 0) {
                        new_type_index = NODE_INDEX_NULL;
                } else {
-                       new_type_index = old_type_index - 1;
+                       if (old_type->type_class == RCU_JA_POOL) {
+                               /*
+                                * For pool type, try redistributing
+                                * into a different distribution of same
+                                * size if we have not reached limits.
+                                */
+                               if (shadow_node->nr_child - 1 < old_type->min_child) {
+                                       new_type_index = old_type_index - 1;
+                               } else {
+                                       new_type_index = old_type_index;
+                               }
+                       } else {
+                               new_type_index = old_type_index - 1;
+                       }
                }
                break;
        default:
@@ -958,7 +1269,8 @@ retry:             /* for fallback */
                        switch (new_type->nr_pool_order) {
                        case 1:
                        {
-                               unsigned int node_distrib_bitsel = 0;
+                               unsigned int node_distrib_bitsel;
+
                                node_distrib_bitsel =
                                        ja_node_sum_distribution_1d(mode, ja,
                                                old_type_index, old_type,
@@ -972,10 +1284,18 @@ retry:           /* for fallback */
                        }
                        case 2:
                        {
-                               /* TODO: pool order 2 in 2d */
+                               unsigned int node_distrib_bitsel[2];
+
+                               ja_node_sum_distribution_2d(mode, ja,
+                                       old_type_index, old_type,
+                                       old_node, shadow_node,
+                                       n, child_node_flag,
+                                       nullify_node_flag_ptr,
+                                       node_distrib_bitsel);
                                assert(!((unsigned long) new_node & JA_POOL_1D_MASK));
                                assert(!((unsigned long) new_node & JA_POOL_2D_MASK));
-                               new_node_flag = ja_node_flag(new_node, new_type_index);
+                               new_node_flag = ja_node_flag_pool_2d(new_node,
+                                       new_type_index, node_distrib_bitsel);
                                break;
                        }
                        default:
@@ -999,7 +1319,7 @@ retry:             /* for fallback */
                new_node_flag = NULL;
        }
 
-       assert(mode != JA_RECOMPACT_ADD || old_type->type_class != RCU_JA_PIGEON);
+       assert(mode != JA_RECOMPACT_ADD_NEXT || old_type->type_class != RCU_JA_PIGEON);
 
        if (new_type_index == NODE_INDEX_NULL)
                goto skip_copy;
@@ -1065,7 +1385,7 @@ retry:            /* for fallback */
                break;
        }
        case RCU_JA_NULL:
-               assert(mode == JA_RECOMPACT_ADD);
+               assert(mode == JA_RECOMPACT_ADD_NEXT);
                break;
        case RCU_JA_PIGEON:
        {
@@ -1099,7 +1419,7 @@ retry:            /* for fallback */
        }
 skip_copy:
 
-       if (mode == JA_RECOMPACT_ADD) {
+       if (mode == JA_RECOMPACT_ADD_NEXT || mode == JA_RECOMPACT_ADD_SAME) {
                /* add node */
                ret = _ja_node_set_nth(new_type, new_node, new_node_flag,
                                new_shadow_node,
@@ -1109,6 +1429,14 @@ skip_copy:
                }
                assert(!ret);
        }
+
+       if (fallback) {
+               dbg_printf("Using fallback for %u children, node type index: %u, mode %s\n",
+                       new_shadow_node->nr_child, old_type_index, mode == JA_RECOMPACT_ADD_NEXT ? "add_next" :
+                               (mode == JA_RECOMPACT_DEL ? "del" : "add_same"));
+               uatomic_inc(&node_fallback_count_distribution[new_shadow_node->nr_child]);
+       }
+
        /* Return pointer to new recompacted node through old_node_flag_ptr */
        *old_node_flag_ptr = new_node_flag;
        if (old_node) {
@@ -1138,7 +1466,68 @@ fallback_toosmall:
                        RCUJA_SHADOW_CLEAR_FREE_NODE);
        assert(!ret);
 
-       /* Choose fallback type: pigeon */
+       switch (mode) {
+       case JA_RECOMPACT_ADD_SAME:
+               /*
+                * JA_RECOMPACT_ADD_SAME is only triggered if a linear
+                * node within a pool has unused entries. It should
+                * therefore _never_ be too small.
+                */
+               //TODO assert(0);
+               break;
+       case JA_RECOMPACT_ADD_NEXT:
+       {
+               const struct cds_ja_type *next_type;
+
+               /*
+                * Recompaction attempt on add failed. Should only
+                * happen if target node type is pool. Caused by
+                * hard-to-split distribution. Recompact using the next
+                * distribution size.
+                */
+               assert(new_type->type_class == RCU_JA_POOL);
+               next_type = &ja_types[new_type_index + 1];
+               /*
+                * Try going to the next pool size if our population
+                * fits within its range. This is not flagged as a
+                * fallback.
+                */
+               if (shadow_node->nr_child + 1 >= next_type->min_child
+                               && shadow_node->nr_child + 1 <= next_type->max_child) {
+                       new_type_index++;
+                       goto retry;
+               } else {
+                       new_type_index++;
+                       dbg_printf("Add fallback to type %d\n", new_type_index);
+                       uatomic_inc(&ja->nr_fallback);
+                       fallback = 1;
+                       goto retry;
+               }
+               break;
+       }
+       case JA_RECOMPACT_DEL:
+               /*
+                * Recompaction attempt on delete failed. Should only
+                * happen if target node type is pool. This is caused by
+                * a hard-to-split distribution. Recompact on same node
+                * size, but flag current node as "fallback" to ensure
+                * we don't attempt recompaction before some activity
+                * has reshuffled our node.
+                */
+               assert(new_type->type_class == RCU_JA_POOL);
+               new_type_index = old_type_index;
+               dbg_printf("Delete fallback keeping type %d\n", new_type_index);
+               uatomic_inc(&ja->nr_fallback);
+               fallback = 1;
+               goto retry;
+       default:
+               assert(0);
+               return -EINVAL;
+       }
+
+       /*
+        * Last resort fallback: pigeon.
+        */
        new_type_index = (1UL << JA_TYPE_BITS) - 1;
        dbg_printf("Fallback to type %d\n", new_type_index);
        uatomic_inc(&ja->nr_fallback);
@@ -1171,13 +1560,13 @@ int ja_node_set_nth(struct cds_ja *ja,
                        n, child_node_flag);
        switch (ret) {
        case -ENOSPC:
-               /* Not enough space in node, need to recompact. */
-               ret = ja_node_recompact(JA_RECOMPACT_ADD, ja, type_index, type, node,
+               /* Not enough space in node, need to recompact to next type. */
+               ret = ja_node_recompact(JA_RECOMPACT_ADD_NEXT, ja, type_index, type, node,
                                shadow_node, node_flag, n, child_node_flag, NULL);
                break;
        case -ERANGE:
                /* Node needs to be recompacted. */
-               ret = ja_node_recompact(JA_RECOMPACT, ja, type_index, type, node,
+               ret = ja_node_recompact(JA_RECOMPACT_ADD_SAME, ja, type_index, type, node,
                                shadow_node, node_flag, n, child_node_flag, NULL);
                break;
        }
@@ -1206,9 +1595,9 @@ int ja_node_clear_ptr(struct cds_ja *ja,
        node = ja_node_ptr(*parent_node_flag_ptr);
        type_index = ja_node_type(*parent_node_flag_ptr);
        type = &ja_types[type_index];
-       ret = _ja_node_clear_ptr(type, node, shadow_node, node_flag_ptr, n);
+       ret = _ja_node_clear_ptr(type, node, *parent_node_flag_ptr, shadow_node, node_flag_ptr, n);
        if (ret == -EFBIG) {
-               /* Should to try recompaction. */
+               /* Should try recompaction. */
                ret = ja_node_recompact(JA_RECOMPACT_DEL, ja, type_index, type, node,
                                shadow_node, parent_node_flag_ptr, n, NULL,
                                node_flag_ptr);
@@ -1985,6 +2374,20 @@ void rcuja_free_all_children(struct cds_ja_shadow_node *shadow_node,
        }
 }
 
+static
+void print_debug_fallback_distribution(void)
+{
+       int i;
+
+       fprintf(stderr, "Fallback node distribution:\n");
+       for (i = 0; i < JA_ENTRY_PER_NODE; i++) {
+               if (!node_fallback_count_distribution[i])
+                       continue;
+               fprintf(stderr, "       %3u: %4lu\n",
+                       i, node_fallback_count_distribution[i]);
+       }
+}
+
 /*
  * There should be no more concurrent add to the judy array while it is
  * being destroyed (ensured by the caller).
@@ -2004,6 +2407,7 @@ int cds_ja_destroy(struct cds_ja *ja,
                fprintf(stderr,
                        "[warning] RCU Judy Array used %lu fallback node(s)\n",
                        uatomic_read(&ja->nr_fallback));
+       print_debug_fallback_distribution();
        free(ja);
        return 0;
 }
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