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

[Andi Kleen <ak@suse.de>]

Christoph Lameter <clameter@sgi.com> writes:
>   and why other approaches have not worked.
> o Instead of zero_page(p,order) extend clear_page to take second argument
> o Update all architectures to accept second argument for clear_pages

Sorry if there was a miscommunication, but ...
> 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.

So the two changes don't really make sense.

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. 

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.


> 2. Hardware support for offloading zeroing from the cpu. This avoids
> the invalidation of the cpu caches by extensive zeroing operations.
> 
> The result is a significant increase of the page fault performance even for
> single threaded applications:
[...]

How about some numbers on i386? 


-Andi