Hi Meir Could you test this patch, if it improves performance for your case?
Mikulas From: Mikulas Patocka <mpato...@redhat.com> There was reported performance degradation when the shadow map contained too many entries [1]. The shadow map uses 256-bucket hash with linear lists - when there are too many entries, it has quadratic complexity. Meir Elisha proposed to add a module parameter that could configure the size of the hash array - however, this is not ideal because users don't know that they should increase the parameter when they get bad performance. This commit replaces the linear lists with rb-trees (so that there's a hash of rb-trees), they have logarithmic complexity, so it solves the performance degradation. Link: https://patchwork.kernel.org/project/dm-devel/patch/20241014134944.1264991-1-meir.eli...@volumez.com/ [1] Reported-by: Meir Elisha <meir.eli...@volumez.com> Signed-off-by: Mikulas Patocka <mpato...@redhat.com> --- drivers/md/persistent-data/dm-transaction-manager.c | 54 +++++++++++++------- 1 file changed, 37 insertions(+), 17 deletions(-) Index: linux-2.6/drivers/md/persistent-data/dm-transaction-manager.c =================================================================== --- linux-2.6.orig/drivers/md/persistent-data/dm-transaction-manager.c 2024-11-05 12:11:40.000000000 +0100 +++ linux-2.6/drivers/md/persistent-data/dm-transaction-manager.c 2025-01-15 14:18:25.000000000 +0100 @@ -13,6 +13,7 @@ #include <linux/export.h> #include <linux/mutex.h> #include <linux/hash.h> +#include <linux/rbtree.h> #include <linux/slab.h> #include <linux/device-mapper.h> @@ -77,7 +78,7 @@ static void prefetch_issue(struct prefet /*----------------------------------------------------------------*/ struct shadow_info { - struct hlist_node hlist; + struct rb_node node; dm_block_t where; }; @@ -95,7 +96,7 @@ struct dm_transaction_manager { struct dm_space_map *sm; spinlock_t lock; - struct hlist_head buckets[DM_HASH_SIZE]; + struct rb_root buckets[DM_HASH_SIZE]; struct prefetch_set prefetches; }; @@ -106,14 +107,22 @@ static int is_shadow(struct dm_transacti { int r = 0; unsigned int bucket = dm_hash_block(b, DM_HASH_MASK); - struct shadow_info *si; + struct rb_node **node; spin_lock(&tm->lock); - hlist_for_each_entry(si, tm->buckets + bucket, hlist) - if (si->where == b) { + node = &tm->buckets[bucket].rb_node; + while (*node) { + struct shadow_info *si = + rb_entry(*node, struct shadow_info, node); + if (b == si->where) { r = 1; break; } + if (b < si->where) + node = &si->node.rb_left; + else + node = &si->node.rb_right; + } spin_unlock(&tm->lock); return r; @@ -130,30 +139,41 @@ static void insert_shadow(struct dm_tran si = kmalloc(sizeof(*si), GFP_NOIO); if (si) { + struct rb_node **node, *parent; si->where = b; bucket = dm_hash_block(b, DM_HASH_MASK); + spin_lock(&tm->lock); - hlist_add_head(&si->hlist, tm->buckets + bucket); + node = &tm->buckets[bucket].rb_node; + parent = NULL; + while (*node) { + struct shadow_info *si = + rb_entry(*node, struct shadow_info, node); + parent = *node; + if (b < si->where) + node = &si->node.rb_left; + else + node = &si->node.rb_right; + } + rb_link_node(&si->node, parent, node); + rb_insert_color(&si->node, &tm->buckets[bucket]); spin_unlock(&tm->lock); } } static void wipe_shadow_table(struct dm_transaction_manager *tm) { - struct shadow_info *si; - struct hlist_node *tmp; - struct hlist_head *bucket; - int i; + unsigned int i; spin_lock(&tm->lock); for (i = 0; i < DM_HASH_SIZE; i++) { - bucket = tm->buckets + i; - hlist_for_each_entry_safe(si, tmp, bucket, hlist) + while (!RB_EMPTY_ROOT(&tm->buckets[i])) { + struct shadow_info *si = + rb_entry(tm->buckets[i].rb_node, struct shadow_info, node); + rb_erase(&si->node, &tm->buckets[i]); kfree(si); - - INIT_HLIST_HEAD(bucket); + } } - spin_unlock(&tm->lock); } @@ -162,7 +182,7 @@ static void wipe_shadow_table(struct dm_ static struct dm_transaction_manager *dm_tm_create(struct dm_block_manager *bm, struct dm_space_map *sm) { - int i; + unsigned int i; struct dm_transaction_manager *tm; tm = kmalloc(sizeof(*tm), GFP_KERNEL); @@ -176,7 +196,7 @@ static struct dm_transaction_manager *dm spin_lock_init(&tm->lock); for (i = 0; i < DM_HASH_SIZE; i++) - INIT_HLIST_HEAD(tm->buckets + i); + tm->buckets[i] = RB_ROOT; prefetch_init(&tm->prefetches);
