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SubjectRe: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE


On Wed, 18 Apr 2018, Christopher Lameter wrote:

> 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.

But slab_order (and its caller) limits the order to "max_order" and we
want more.

Perhaps slab_order should be dropped and calculate_order totally
rewritten?

Mikulas

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