Re: [RFC] Transparent on-demand memory setup initialization embedded in the (GFP) buddy allocator

[Nathan Zimmer <nzimmer@sgi.com>]

On 06/26/2013 10:35 PM, Daniel J Blueman wrote:
> On Wednesday, June 26, 2013 9:30:02 PM UTC+8, Andrew Morton wrote:
> >
> > On Wed, 26 Jun 2013 11:22:48 +0200 Ingo Molnar <mi...@kernel.org> 
> wrote:
> >
> > > except that on 32 TB
> > > systems we don't spend ~2 hours initializing 8,589,934,592 page 
> heads.
> >
> > That's about a million a second which is crazy slow - even my 
> prehistoric desktop
> > is 100x faster than that.
> >
> > Where's all this time actually being spent?
>
> The complexity of a directory-lookup architecture to make the 
> (intrinsically unscalable) cache-coherency protocol scalable gives you 
> a ~1us roundtrip to remote NUMA nodes.
>
> Probably a lot of time is spent in some memsets, and RMW cycles which 
> are setting page bits, which are intrinsically synchronous, so the 
> initialising core can't get to 12 or so outstanding memory transactions.
>
> Since EFI memory ranges have a flag to state if they are zerod (which 
> may be a fair assumption for memory on non-bootstrap processor NUMA 
> nodes), we can probably collapse the RMWs to just writes.
>
> A normal write will require a coherency cycle, then a fetch and a 
> writeback when it's evicted from the cache. For this purpose, 
> non-temporal writes would eliminate the cache line fetch and give a 
> massive increase in bandwidth. We wouldn't even need a store-fence as 
> the initialising core is the only one online.
>
> Daniel

Could you elaborate a bit more? or suggest a specific area to look at?

After some experiments with trying to just set some fields in the struct 
page directly I haven't been able to produce any improvements.  Of 
course there is lots about the area which I don't have much experience with.

Nate