In an attempt to understand the numbers found when viewing '/proc/slabinfo' on the 2.4 kernel [forum], Doug Ledford read through the source code. He then posted a summary to the lkml, sharing what he'd learned. He says, "I wrote this by looking at the 2.4 kernel sources, so it may not be totally accurate as far as 2.6 is concerned, but since there is no documentation on it at the moment, it's certainly better than what's there." Read on for Doug's full explanation.
Those interested in slabinfo will also want to check out the recently released slabtop utility [story], which provides detailed, real time kernel slab cache information.
From: Doug Ledford [email blocked] Subject: New: Documentation/vm/slabinfo.txt Date: Sat, 01 Nov 2003 07:01:09 -0500 I wrote this by looking at the 2.4 kernel sources, so it may not be totally accurate as far as 2.6 is concerned, but since there is no documentation on it at the moment, it's certainly better than what's there. ------Snip-------- > cat /proc/slabinfo: > inode_cache 423370 423556 512 60482 60508 1 : 248 62 > dentry_cache 435756 436260 128 14526 14542 1 : 504 126 Hmmm...those lines looked suspicious, but I didn't really know how to read the slabinfo output. So I just went and read up on it. For those people like me that didn't know what all these numbers mean, here's my understanding after reading the source: active-objects | allocated-objects | | object-size | | | active-slab-allocations | | | | total-slab-allocations | | | | | alloc-size | | | | | | inode_cache 423370 423556 512 60482 60508 1 : 248 62 | | limit | batch-count active-objects: after creating a slab cache, you allocate your objects out of that slab cache. This is the count of objects you currently have allocated out of the cache. allocated-objects: this is the current total number of objects in the cache. object-size: this is the size of each allocated object. There is overhead to maintaining the cache, so with a 512byte object and a 4096byte page size, you could fit 7 objects in a single page and you would waste 512-slab_overhead bytes per allocation. Slab overhead varies with object size (smaller objects have more objects per allocation and require more overhead to track used vs. unused objects). You can determine how many objects are being put on each allocation chunk by dividing allocated-objects by total-allocations. active-slab-allocs: This is the number of allocations that have at least one of that allocations objects in use. total-slab-allocs: The total number of allocations in the current slab cache. alloc-size: This is the size of each allocation in units of memory pages. Page size is architecture specific, but the most common size is 4k. A couple architectures have an 8k page size, and ia64 can do a 16k page size. Each allocation for the cache is alloc-size * arch_page_size bytes at a time, and total memory used by this particular slab cache is total_slab_allocs * alloc_size * arch_page_size. The last 2 items are SMP specific and don't show up at all on UP kernels. On SMP machines, the slab cache will keep a per CPU cache of objects so that an object freed on CPU0 will be reused on CPU0 instead of CPU1 if possible. This improves cache performance on SMP systems greatly. limit: This is the limit on the number of free objects that can be stored in the per-CPU free list for this slab cache. batch-count: On SMP systems, when we refill the available object list, instead of doing one object at a time, we do batch-count objects at a time. One last thing, if slab statistics are enabled, then you'll get more numbers on each line and the lines will look like this: UP name active-objects total-objects object-size active-allocs total-allocs alloc-size: high-size num-allocations times-grown allocs-reaped errors SMP name active-objects total-objects object-size active-allocs total-allocs alloc-size: high-size num-allocations times-grown allocs-reaped errors: limit batch-count: alloc-hits alloc-misses free-hits free-misses For the most part, the statistics numbers are pretty self explanatory, so I won't bother with them. HTH -- Doug Ledford [email blocked] 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606