[PATCH 4/4] kern: optimize the layout of struct kmem_cache

[Justus Winter]

* kern/slab.h (struct kmem_cache): Reorder the fields so that all hot
fields are within the first cache line.
---
 kern/slab.h | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/kern/slab.h b/kern/slab.h
index fd65893..457dee4 100644
--- a/kern/slab.h
+++ b/kern/slab.h
@@ -157,6 +157,10 @@ typedef void (*kmem_slab_free_fn_t)(vm_offset_t, 
vm_size_t);
  * Cache of objects.
  *
  * Locking order : cpu_pool -> cache. CPU pools locking is ordered by CPU ID.
+ *
+ * Currently, SLAB_USE_CPU_POOLS is not defined.  KMEM_CACHE_NAME_SIZE
+ * is chosen so that the struct fits into two cache lines.  The first
+ * cache line contains all hot fields.
  */
 struct kmem_cache {
 #if SLAB_USE_CPU_POOLS
@@ -172,19 +176,21 @@ struct kmem_cache {
     struct list free_slabs;
     struct rbtree active_slabs;
     int flags;
+    size_t bufctl_dist; /* Distance from buffer to bufctl   */
+    size_t slab_size;
+    unsigned long bufs_per_slab;
+    unsigned long nr_objs;  /* Number of allocated objects */
+    unsigned long nr_free_slabs;
+    kmem_cache_ctor_t ctor;
+    /* All fields below are cold  */
     size_t obj_size;    /* User-provided size */
+    /* Assuming ! SLAB_USE_CPU_POOLS, here is the cacheline boundary */
     size_t align;
     size_t buf_size;    /* Aligned object size  */
-    size_t bufctl_dist; /* Distance from buffer to bufctl   */
-    size_t slab_size;
     size_t color;
     size_t color_max;
-    unsigned long bufs_per_slab;
-    unsigned long nr_objs;  /* Number of allocated objects */
     unsigned long nr_bufs;  /* Total number of buffers */
     unsigned long nr_slabs;
-    unsigned long nr_free_slabs;
-    kmem_cache_ctor_t ctor;
     kmem_slab_alloc_fn_t slab_alloc_fn;
     kmem_slab_free_fn_t slab_free_fn;
     char name[KMEM_CACHE_NAME_SIZE];
-- 
1.8.5.2