Re: ZERO_PAGE refcounting causes cache line bouncing

[William Lee Irwin III <wli@holomorphy.com>]

On Fri, Mar 16, 2007 at 03:17:39PM -0700, Christoph Lameter wrote:
> We have issues with ZERO_PAGE refcounting causing severe cacheline 
> bouncing. ZERO_PAGES are mapped into multiple processes running on 
> multiple nodes. Refcounter modifications therefore have to acquire a 
> remote exclusive cacheline.
> Could we somehow fix this? There are a couple of ways to do this:
> 1. No refcounting on reserved pages in the VM. ZERO_PAGEs are
>    reserved and there is no point in refcounting them since they
>    will not go away.
> 2. Having a percpu or pernode ZERO_PAGE?
>    May be a simpler solution but then we still may have issues
>    if the ZERO_PAGE gets "freed" from other processors/ nodes.

It's dumb to refcount the zero page. Someone should've noticed this
when the PG_reserved patches went in. I can't think of an easy way
around this apart from a backout. OTOH it's a simple matter of
programming to arrange for it without a backout.

Provisions should be made for per-node zero pages in addition to this.
AFAICT the primary thing needed is to wrap checks for a page being a
zero page with some testing function instead of using a raw equality
check. This is above and beyond solving the mere zero page refcount
problem; I'm saying that both proposals should be done even though only
one is needed to resolve the bouncing issue.

I guess this is an "ack of the concept" of sorts. None of this is so
involved that I should jump on it and try to get a patch out ahead of
you.


-- wli

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