Re: objrmap-core-1 (rmap removal for file mappings to avoid 4:4 in <=16G machines)

From: William Lee Irwin III
Date: Tue Mar 09 2004 - 03:46:37 EST

On Tue, Mar 09, 2004 at 09:31:03AM +0100, Ingo Molnar wrote:
> with rmap we do have the ability to make it truly O(1) all across, by
> making the pte chains a double linked list. Moreover, we can freely
> reduce the rmap overhead (both the memory, algorithmic and locking
> overhead) by increasing the page size - a natural thing to do on big
> boxes anyway. The increasing of the page size also linearly _reduces_
> the RAM footprint of rmap. So rmap and pgcl are a natural fit and the
> thing of the future.
> now, the linear searching of vmas does not reduce with increased
> page-size. In fact, it will increase in time as the sharing factor
> increases.

This is getting bandied about rather frequently. I should make some
kind of attack on an implementation. The natural implementation is
to add one pte per contiguous and aligned group of PAGE_MMUCOUNT ptes
to the pte_chain and search the area surrounding any pte_chain element.

But the linear search you're pointing at is unnecessary to begin with.
Only the single nonlinear mappings' pte needs to be added to the
pte_chain there; one need only also scan the vma lists at reclaim-time.
This would also make page_convert_anon() a misnomer and SetPageAnon()
on nonlinearly-mapped file-backed pages a bug.

-- wli
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