Re: Prezeroing V2 [0/3]: Why and When it works

[Christoph Lameter]

On Thu, 23 Dec 2004, Andi Kleen wrote:

> > 1. Aggregating zeroing operations to only apply to pages of higher order,
> > which results in many pages that will later become order 0 to be
> > zeroed in one go. For that purpose the existing clear_page function is
> > extended and made to take an additional argument specifying the order of
> > the page to be cleared.
>
> But if you do that you should really use a separate function that
> can use cache bypassing stores.
>
> Normal clear_page cannot use that because it would be a loss
> when the data is soon used.

Clear_page is used both in the cache hot and no cache wanted case now.

> So the two changes don't really make sense.

Which two changes?

If an arch can do zeroing without touching the cpu caches then that can
be done with a zero driver.

> Also I must say I'm still suspicious regarding your heuristic
> to trigger gang faulting - with bad luck it could lead to a lot
> more memory usage to specific applications that do very sparse
> usage of memory.

Gang faulting is not part of this patch. Please keep the issues separate.

> There should be at least an madvise flag to turn it off and a sysctl
> and it would be better to trigger only on a longer sequence of
> consecutive faulted pages.

Again this is not related to this patchset. Look at the V13 of the page
fault scalability patch and you will find a /proc/sys/vm setting to
manipulate things. This is V2 of the prezeroing patch.

> How about some numbers on i386?

Umm. Yeah. I only have smallish i386 machines here. Maybe next year ;-)
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