On Mon, Sep 09, 2013 at 02:44:03PM +0000, Christoph Lameter wrote:
> On Mon, 9 Sep 2013, Joonsoo Kim wrote:
>
> > 32 byte is not minimum object size, minimum *kmalloc* object size
> > in default configuration. There are some slabs that their object size is
> > less than 32 byte. If we have a 8 byte sized kmem_cache, it has 512 objects
> > in 4K page.
>
> As far as I can recall only SLUB supports 8 byte objects. SLABs mininum
> has always been 32 bytes.
No.
There are many slabs that their object size are less than 32 byte.
And I can also create a 8 byte sized slab in my kernel with SLAB.
js1304@js1304-P5Q-DELUXE:~/Projects/remote_git/linux$ sudo cat /proc/slabinfo | awk '{if($4 < 32) print $0}'
slabinfo - version: 2.1
ecryptfs_file_cache 0 0 16 240 1 : tunables 120 60 8 : slabdata 0 0 0
jbd2_revoke_table_s 2 240 16 240 1 : tunables 120 60 8 : slabdata 1 1 0
journal_handle 0 0 24 163 1 : tunables 120 60 8 : slabdata 0 0 0
revoke_table 0 0 16 240 1 : tunables 120 60 8 : slabdata 0 0 0
scsi_data_buffer 0 0 24 163 1 : tunables 120 60 8 : slabdata 0 0 0
fsnotify_event_holder 0 0 24 163 1 : tunables 120 60 8 : slabdata 0 0 0
numa_policy 3 163 24 163 1 : tunables 120 60 8 : slabdata 1 1 0
>
> > Moreover, we can configure slab_max_order in boot time so that we can't know
> > how many object are in a certain slab in compile time. Therefore we can't
> > decide the size of the index in compile time.
>
> You can ignore the slab_max_order if necessary.
>
> > I think that byte and short int sized index support would be enough, but
> > it should be determined at runtime.
>
> On x86 f.e. it would add useless branching. The branches are never taken.
> You only need these if you do bad things to the system like requiring
> large contiguous allocs.
As I said before, since there is a possibility that some runtime loaded modules
use a 8 byte sized slab, we can't determine index size in compile time. Otherwise
we should always use short int sized index and I think that it is worse than
adding a branch.
Thanks.
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