Re: [RFC] another way to speed up fake numa node page_alloc

[David Rientjes <rientjes@cs.washington.edu>]

On Tue, 3 Oct 2006, Paul Jackson wrote:

> pj, responding to David:
> > > With NODES_SHIFT equal to 10 as you recommend, you can't get away with an 
> > > unsigned short there. 
> > 
> > Apparently it's time for me to be a stupid git again.  That's ok; I'm
> > getting quite accustomed to it.
> > 
> > Could you spell out exactly why I can't get away with an unsigned short
> > node_id if NODES_SHIFT is 10?
> 
> 
> Is this still in your queue to respond to, David?
> 
> I'm still curious as to why I can't get away with an unsigned short there.
> 

Because it's unnecessary.  On my 4G machine with numa=fake=256, each of 
these node_id arrays is going to be 1.5K.  You could get away with the 
exact same behavior with using a u8 or unsigned char.  There's no reason 
to support anything greater than a shift of 8 since NUMA emulation is 
_only_ available on x86_64 and doesn't even work right as it stands in the 
current mainline so that you could boot my machine with anything more than 
numa=fake=8.

If you are going to abstract this functionality to other architectures or 
even generically I would suggest following Magnus Damm's example and 
creating a NODES_SHIFT_HW instead that would limit the number of numa=fake 
nodes.  There is simply no reason for this to be greater than 8 (even a 
128G machine with numa=fake=256 would have 512M nodes).

Secondly, the entire node_id lookup is redundant on x86_64 in the first 
place (see arch/x86_64/mm/numa.c and include/asm-x86_64/mmzone.h for 
memnodemap).  The only thing that is being sped-up with your node_id array 
in each zonelist_faster is moving this calculation from two steps to one 
step; since the mainline implementation today are both inline functions I 
think the improvement is minimal.

		David

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