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SubjectRe: [PATCH 35/35] autonuma: page_autonuma
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On Fri, 2012-05-25 at 19:02 +0200, Andrea Arcangeli wrote:
> Move the AutoNUMA per page information from the "struct page" to a
> separate page_autonuma data structure allocated in the memsection
> (with sparsemem) or in the pgdat (with flatmem).
>
> This is done to avoid growing the size of the "struct page" and the
> page_autonuma data is only allocated if the kernel has been booted on
> real NUMA hardware (or if noautonuma is passed as parameter to the
> kernel).
>

Argh, please fold this change back into the series proper. If you want
to keep it.. as it is its not really an improvement IMO, see below.

> +struct page_autonuma {
> + /*
> + * FIXME: move to pgdat section along with the memcg and allocate
> + * at runtime only in presence of a numa system.
> + */
> + /*
> + * To modify autonuma_last_nid lockless the architecture,
> + * needs SMP atomic granularity < sizeof(long), not all archs
> + * have that, notably some alpha. Archs without that requires
> + * autonuma_last_nid to be a long.
> + */

Looking at arch/alpha/include/asm/xchg.h it looks to have that just
fine, so maybe we simply don't support SMP on those early Alphas that
had that weirdness.

> +#if BITS_PER_LONG > 32
> + int autonuma_migrate_nid;
> + int autonuma_last_nid;
> +#else
> +#if MAX_NUMNODES >= 32768
> +#error "too many nodes"
> +#endif
> + /* FIXME: remember to check the updates are atomic */
> + short autonuma_migrate_nid;
> + short autonuma_last_nid;
> +#endif
> + struct list_head autonuma_migrate_node;
> +
> + /*
> + * To find the page starting from the autonuma_migrate_node we
> + * need a backlink.
> + */
> + struct page *page;
> +};

This makes a shadow page frame of 32 bytes per page, or ~0.8% of memory.
This isn't in fact an improvement.

The suggestion done by Rik was to have something like a sqrt(nr_pages)
(?) scaled array of such things containing the list_head and page
pointer -- and leave the two nids in the regular page frame. Although I
think you've got to fight the memcg people over that last word in struct
page.

That places a limit on the amount of pages that can be in migration
concurrently, but also greatly reduces the memory overhead.


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