[patch 2/4] mm: writeback: distribute write pages across allowable zones

[Rik van Riel]

On 09/20/2011 09:45 AM, Johannes Weiner wrote:
> This patch allows allocators to pass __GFP_WRITE when they know in
> advance that the allocated page will be written to and become dirty
> soon.  The page allocator will then attempt to distribute those
> allocations across zones, such that no single zone will end up full of
> dirty, and thus more or less, unreclaimable pages.
>
> The global dirty limits are put in proportion to the respective zone's
> amount of dirtyable memory and allocations diverted to other zones
> when the limit is reached.
>
> For now, the problem remains for NUMA configurations where the zones
> allowed for allocation are in sum not big enough to trigger the global
> dirty limits, but a future approach to solve this can reuse the
> per-zone dirty limit infrastructure laid out in this patch to have
> dirty throttling and the flusher threads consider individual zones.
>
> Signed-off-by: Johannes Weiner<jweiner at redhat.com>

Reviewed-by: Rik van Riel <riel at redhat.com>

The amount of work done in a __GFP_WRITE allocation looks
a little daunting, but doing that a million times probably
outweighs waiting on the disk even once, so...