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SubjectRe: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE
On Tue, 17 Apr 2018, Mikulas Patocka wrote:

> I can make a slub-only patch with no extra flag (on a freshly booted
> system it increases only the order of caches "TCPv6" and "sighand_cache"
> by one - so it should not have unexpected effects):
>
> Doing a generic solution for slab would be more comlpicated because slab
> assumes that all slabs have the same order, so it can't fall-back to
> lower-order allocations.

Well again SLAB uses compound pages and thus would be able to detect the
size of the page. It may be some work but it could be done.

>
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c 2018-04-17 19:59:49.000000000 +0200
> +++ linux-2.6/mm/slub.c 2018-04-17 20:58:23.000000000 +0200
> @@ -3252,6 +3252,7 @@ static inline unsigned int slab_order(un
> static inline int calculate_order(unsigned int size, unsigned int reserved)
> {
> unsigned int order;
> + unsigned int test_order;
> unsigned int min_objects;
> unsigned int max_objects;
>
> @@ -3277,7 +3278,7 @@ static inline int calculate_order(unsign
> order = slab_order(size, min_objects,
> slub_max_order, fraction, reserved);
> if (order <= slub_max_order)
> - return order;
> + goto ret_order;
> fraction /= 2;
> }
> min_objects--;
> @@ -3289,15 +3290,25 @@ static inline int calculate_order(unsign
> */
> order = slab_order(size, 1, slub_max_order, 1, reserved);

The slab order is determined in slab_order()

> if (order <= slub_max_order)
> - return order;
> + goto ret_order;
>
> /*
> * Doh this slab cannot be placed using slub_max_order.
> */
> order = slab_order(size, 1, MAX_ORDER, 1, reserved);
> - if (order < MAX_ORDER)
> - return order;
> - return -ENOSYS;
> + if (order >= MAX_ORDER)
> + return -ENOSYS;
> +
> +ret_order:
> + for (test_order = order + 1; test_order < MAX_ORDER; test_order++) {
> + unsigned long order_objects = ((PAGE_SIZE << order) - reserved) / size;
> + unsigned long test_order_objects = ((PAGE_SIZE << test_order) - reserved) / size;
> + if (test_order_objects > min(32, MAX_OBJS_PER_PAGE))
> + break;
> + if (test_order_objects > order_objects << (test_order - order))
> + order = test_order;
> + }
> + return order;

Could yo move that logic into slab_order()? It does something awfully
similar.

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