Kernel Memory Caches

The kernel memory allocator divides the memory it manages into a set of caches. All allocations are supplied from these caches, which are represented by the kmem_cache_t data structure. Each cache has a fixed buffer size, which represents the maximum allocation size satisfied by that cache. Each cache has a string name indicating the type of data it manages.

Some kernel memory caches are special purpose and are initialized to allocate only a particular kind of data structure. An example of this is the “thread_cache,” which allocates only structures of type kthread_t. Memory from these caches is allocated to clients by the kmem_cache_alloc() function and freed by the kmem_cache_free() function.

kmem_cache_alloc() and kmem_cache_free() are not public DDI interfaces. Do NOT write code that relies on them, because they are subject to change or removal in future releases of Oracle Solaris.

Caches whose name begins with “kmem_alloc_” implement the kernel's general memory allocation scheme. These caches provide memory to clients of kmem_alloc(9F) and kmem_zalloc(9F). Each of these caches satisfies requests whose size is between the buffer size of that cache and the buffer size of the next smallest cache. For example, the kernel has kmem_alloc_8 and kmem_alloc_16 caches. In this case, the kmem_alloc_16 cache handles all client requests for 9-16 bytes of memory. Remember that the size of each buffer in the kmem_alloc_16 cache is 16 bytes, regardless of the size of the client request. In a 14 byte request, two bytes of the resulting buffer are unused, since the request is satisfied from the kmem_alloc_16 cache.

The last set of caches are those used internally by the kernel memory allocator for its own bookkeeping. These include those caches whose names start with “kmem_magazine_” or “kmem_va_”, the kmem_slab_cache, the kmem_bufctl_cache and others.