Greencreek snoop-filter

[Kilian CAVALOTTI]

On Monday 26 March 2007 01:07:09 am Peter Kjellstrom wrote:
> > 1. is there a way to know if it's enabled or not from the OS?
>
> The bits are all available via pci-registers:
>  setpci -s 10.0 f0.b
>
> This just reads a pci-register so it should be safe enough. 7c is snoop
> off, 78 is snoop on.

Very good, thanks a lot.

> > 2. is there a way to enable it without going to the BIOS? I think
> > that "omconfig chassis biossetup" does not allow this action. Is there
> > a plan to add this feature?
>
> Turning snoop off like this:
>  setpci -s 10.0 f0.b=7c
>
> has worked ok for me but turning it on (same but =78) has killed the
> system, YMMW...

Same here. Turning it off works, but turning it on is a sure way to kill 
the machine. Even pretty badly, since the BMC hangs too. I guess that's 
why this option can not be changed with omconfig... ;)

> > 3; what would be the performance impact of enabling the snoop-filter
> > on memory-intensive jobs?
>
> This has turned out to be best predicted using a dice... Some
> applications get a boot from snoop and some the opposite. For the
> benchmarks I/we have run it seems best to leave snoop off.

After running a couple HPL jobs, I came to the same conclusion: it seems 
that enabling the snoop-filter reduces the number of flop/s by a few 
percents.

Thanks for the info,
Cheers,
-- 
Kilian