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SubjectRe: [PATCH 00/16] slab: overload struct slab over struct page to reduce memory usage
On Thu, Aug 22, 2013 at 04:47:25PM +0000, Christoph Lameter wrote:
> On Thu, 22 Aug 2013, Joonsoo Kim wrote:
>
> > And this patchset change a management method of free objects of a slab.
> > Current free objects management method of the slab is weird, because
> > it touch random position of the array of kmem_bufctl_t when we try to
> > get free object. See following example.
>
> The ordering is intentional so that the most cache hot objects are removed
> first.

Yes, I know.

>
> > To get free objects, we access this array with following pattern.
> > 6 -> 3 -> 7 -> 2 -> 5 -> 4 -> 0 -> 1 -> END
>
> Because that is the inverse order of the objects being freed.
>
> The cache hot effect may not be that significant since per cpu and per
> node queues have been aded on top. So maybe we do not be so cache aware
> anymore when actually touching struct slab.

I don't change the ordering, I just change how we store that order to
reduce cache footprint. We can simply implement this order via stack.

Assume indexes of free order are 1 -> 0 -> 4.
Currently, this order is stored in very complex way like below.

struct slab's free = 4
kmem_bufctl_t array: 1 END ACTIVE ACTIVE 0

If we allocate one object, we access slab's free and index 4 of
kmem_bufctl_t array.

struct slab's free = 0
kmem_bufctl_t array: 1 END ACTIVE ACTIVE ACTIVE
<we get object at index 4>

And then,

struct slab's free = 1
kmem_bufctl_t array: ACTIVE END ACTIVE ACTIVE ACTIVE
<we get object at index 0>

And then,

struct slab's free = END
kmem_bufctl_t array: ACTIVE ACTIVE ACTIVE ACTIVE ACTIVE
<we get object at index 0>

Following is newly implementation (stack) in same situation.

struct slab's free = 0
kmem_bufctl_t array: 4 0 1

To get an one object,

struct slab's free = 1
kmem_bufctl_t array: dummy 0 1
<we get object at index 4>

And then,

struct slab's free = 2
kmem_bufctl_t array: dummy dummy 1
<we get object at index 0>

struct slab's free = 3
kmem_bufctl_t array: dummy dummy dummy
<we get object at index 1>

The order of returned object is same as previous algorithm.
However this algorithm sequentially accesses kmem_bufctl_t array,
instead of randomly access. This is an advantage of this patch.

Thanks.


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