Re: SLUB

[Mark Seger <Mark.Seger@hp.com>]

Now that I've had some more time to think about this and play around 
with the slabinfo tool I fear my problem had getting my head wrapped 
around the terminology, but that's my problem.  Since there are entries 
called object_size, objs_per_slab and slab_size I would have thought 
that object_size*objects_per_slab=slab_size but that clearly isn't the 
case.  Since slabs are allocated in pages, the actual size of the slabs 
is always a multiple of the page_size (actually by a power of 2) and 
that's why I see calculations in slabinfo like page_size << order, but I 
guess I'm still not sure what the  actual definition of 'order' actually is.

Anyhow, when I run slabinfo and see the following entry

Slabcache: skbuff_fclone_cache   Aliases:  0 Order :  0 Objects: 25
** Hardware cacheline aligned

Sizes (bytes)     Slabs              Debug                Memory
------------------------------------------------------------------------
Object :     420  Total  :       4   Sanity Checks : Off  Total:   16384
SlabObj:     448  Full   :       0   Redzoning     : Off  Used :   10500
SlabSiz:    4096  Partial:       0   Poisoning     : Off  Loss :    5884
Loss   :      28  CpuSlab:       4   Tracking      : Off  Lalig:     700
Align  :       0  Objects:       9   Tracing       : Off  Lpadd:     256

according to the entries under /sys/slabs/skbuff_fclone_cache it looks 
like the slab_size field is being reported above as 'SlabObj' and 
objs_per_slab is being reported as 'Objects' and as I mentioned above, 
SlabSiz is based on 'order'.

Anyhow, as I understand what's going on at a very high level, memory is 
reserved for use as slabs (which themselves are multiples of pages) and 
processes allocate objects from within slabs as they need them.  
Therefore the 2 high-level numbers that seem of interest from a memory 
usage perspective are the memory allocated and the amount in use.  I 
think these are the "Total" and "Used" fields in slabinfo.

Total = page_size << order

As for 'Used' that looks to be a straight calculation of objects * 
object_size

The Slabs field in /proc/meminfo is the total of the individual 'Total's...

Stay tuned and at some point I'll have support in collectl for reporting 
total/allocated usage by slab in collectl, though perhaps I'll post a 
'proposal' first in the hopes of getting some constructive feedback as I 
want to present useful information rather than that columns of numbers.

-mark


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