Frequently used objects in the Linux kernel (buffer heads, inodes, dentries, etc.) have their own cache. The file /proc/slabinfo gives statistics. For example:
% cat /proc/slabinfo
slabinfo - version: 1.1
kmem_cache 6078 100 2 2 1
blkdev_requests5120 512096 128 128 1
mnt_cache 204096 1 1 1
inode_cache 7005 14792 480 1598 1849 1
dentry_cache 5469 5880 128 183 196 1
filp 726 76096 19 19 1
buffer_head 67131 7124096 1776 1781 1
vm_area_struct 1204 165264 23 28 1
size-8192 117 8192 1 17 2
size-4096 4173 4096 41 73 1
For each slab cache, the cache name, the number of currently active objects, the total number of available objects, the size of each object in bytes, the number of pages with at least one active object, the total number of allocated pages, and the number of pages per slab are given.
Note that because of object alignment and slab cache overhead, objects are not normally packed tightly into pages. Pages with even one in-use object are considered in-use and cannot be freed.
Kernels compiled with slab cache statistics will also have "(statistics)" in the first line of output, and will have 5 additional columns, namely: the high water mark of active objects; the number of times objects have been allocated; the number of times the cache has grown (new pages added to this cache); the number of times the cache has been reaped (unused pages removed from this cache); and the number of times there was an error allocating new pages to this cache. If slab cache statistics are not enabled for this kernel, these columns will not be shown.
SMP systems will also have "(SMP)" in the first line of output, and will have two additional columns for each slab, reporting the slab allocation policy for the CPU-local cache (to reduce the need for inter-CPU synchronization when allocating objects from the cache). The first column is the per-CPU limit: the maximum number of objects that will be cached for each CPU. The second column is the batchcount: the maximum number of free objects in the global cache that will be transferred to the per-CPU cache if it is empty, or the number of objects to be returned to the global cache if the per-CPU cache is full.
If both slab cache statistics and SMP are defined, there will be four additional columns, reporting the per-CPU cache statistics. The first two are the per-CPU cache allocation hit and miss counts: the number of times an object was or was not available in the per-CPU cache for allocation. The next two are the per-CPU cache free hit and miss counts: the number of times a freed object could or could not fit within the per-CPU cache limit, before flushing objects to the global cache.
It is possible to tune the SMP per-CPU slab cache limit and batchcount via:
echo "cache_name limit batchcount" > /proc/slabinfo
/proc/slabinfo exists since Linux 2.1.23. SMP per-CPU caches exist since Linux 2.4.0-test3.