Re: [PATCH 35/35] autonuma: page_autonuma

[Peter Zijlstra]

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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