[Tarantool-patches] [PATCH] small: unite the oscillation cache of all mempools

[Maksim Kulis <bokuno@picodata.io>]

The task is to unite the oscillation cache of all mempools.
In the function slab_put_with_order() we check if there is any 
slab of the largest size except the current one, in order to 
avoid oscillation. 
https://github.com/tarantool/small/blob/master/small/slab_cache.c#L349

---
 small/mempool.c | 22 ++++------------------
 small/mempool.h |  6 ------
 2 files changed, 4 insertions(+), 24 deletions(-)

diff --git a/small/mempool.c b/small/mempool.c
index b20a416..54bd58d 100644
--- a/small/mempool.c
+++ b/small/mempool.c
@@ -126,18 +126,9 @@ mslab_free(struct mempool *pool, struct mslab *slab, void *ptr)
 		}
 		mslab_tree_remove(&pool->hot_slabs, slab);
 		slab->in_hot_slabs = false;
-		if (pool->spare > slab) {
-			slab_list_del(&pool->slabs, &pool->spare->slab,
-				      next_in_list);
-			slab_put_with_order(pool->cache, &pool->spare->slab);
-			pool->spare = slab;
-		 } else if (pool->spare) {
-			 slab_list_del(&pool->slabs, &slab->slab,
-				       next_in_list);
-			 slab_put_with_order(pool->cache, &slab->slab);
-		 } else {
-			 pool->spare = slab;
-		 }
+		slab_list_del(&pool->slabs, &slab->slab,
+			      next_in_list);
+		slab_put_with_order(pool->cache, &slab->slab);
 	}
 }
 
@@ -153,7 +144,6 @@ mempool_create_with_order(struct mempool *pool, struct slab_cache *cache,
 	mslab_tree_new(&pool->hot_slabs);
 	pool->first_hot_slab = NULL;
 	rlist_create(&pool->cold_slabs);
-	pool->spare = NULL;
 	pool->objsize = objsize;
 	pool->slab_order = order;
 	/* Total size of slab */
@@ -179,11 +169,7 @@ mempool_alloc(struct mempool *pool)
 {
 	struct mslab *slab = pool->first_hot_slab;
 	if (slab == NULL) {
-		if (pool->spare) {
-			slab = pool->spare;
-			pool->spare = NULL;
-
-		} else if ((slab = (struct mslab *)
+		if ((slab = (struct mslab *)
 			    slab_get_with_order(pool->cache,
 						pool->slab_order))) {
 			mslab_create(slab, pool);
diff --git a/small/mempool.h b/small/mempool.h
index 15d85a4..9d0e162 100644
--- a/small/mempool.h
+++ b/small/mempool.h
@@ -149,12 +149,6 @@ struct mempool
 	 * tree is empty or the allocator runs out of memory.
 	 */
 	struct rlist cold_slabs;
-	/**
-	 * A completely empty slab which is not freed only to
-	 * avoid the overhead of slab_cache oscillation around
-	 * a single element allocation.
-	 */
-	struct mslab *spare;
 	/**
 	 * The size of an individual object. All objects
 	 * allocated on the pool have the same size.
-- 
2.17.1