Re: SLUB

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

It feels like we're closing in on something as O'm getting more 'Right's 
from you than before.  8-)
Just a few more comments/questions to your comments below...

Christoph Lameter wrote:
> On Thu, 27 Dec 2007, Mark Seger wrote:
>
>   
>> 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.
>>     
>
> order is the shift you apply to PAGE_SIZE to get to the allocation size 
> you want. Order 0 = PAGE_SIZE, order 1 = PAGE_SIZE << 1 (PAGE_SIZE *2), 
> order 2 = PAGE_SIZE << 2 (PAGE_SIZE * 4) etc.
>   
I think the thing that was throwing me here for awhile was the name 
'order'.  I thought it meant order in the ordinal sense but clearly it's 
more intended as is 'the power of' sense.
>> 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 is the total memory allocated from the page allocator. There are 4 
> slab allocated with the size of 4096 bytes each. This is 16k.
>
> The used value is the memory that was actually handed out through kmalloc 
> and friends.
>  
>   
>> Total = page_size << order
>>     
>
> Order = 0. So Total would be 4096 << 0 = 4096. Wrong value.
>   
I'm not sure what your 'wong value.  I think it's because I said 
page_size << order instead of (page_size << order ) * number of slabs, 
right?
>> As for 'Used' that looks to be a straight calculation of objects * object_size
>>     
>
> Right.
>
>   
>> The Slabs field in /proc/meminfo is the total of the individual 'Total's...
>>     
>
> Right.
>
>   
>> 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.
>>     
>
> Ahh Great. Thanks for all your work.
>   
now the only assumption is that someone will actually use it!   8-)

one more thing - can I assume order is a constant for a particular type 
of a slab and only need to read it at initialization time?

-mark

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>