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SubjectRe: Celistica with AMD chip-set
Hello Alan and Richard,

I have to advise caution here, as it is currently unconfirmed whether or
not the PCI bridge configuration is "incorrect", and that it has "very
poor PCI performance".
Unless everyone in the whole wide world is setting this value and we are
the only ones who are not, I find it hard to believe that this statement
is not overspeculative.

The proper place for this should be in the BIOS, if it is indeed a true
optimization point.
But until that is positively identified, we should not assume that
applying this globally for everyone is the right thing to do.
As in any assumed optimization for a simgle case, it could potentially
cause performance degradation in somebody else's HBA.

This is a cache optimization.

Have you considered the possibility of this "optimization" causing a
performance hit with Mellanox's PCI implementation?

What about people who have already tailored their device driver to work
well in on this chipset and currently use "read multiple" rather than
"read cacheline". This optimization could potentially cause a slight
degradation of performance for them.

I just propose that we test this change with various card vendors and see
what the real impact is before we jump to the conclusion that this is a
serious performance problem for everybody.

Secondly, if it is the case, then the correct place to put this change is
in the system's BIOS, and having a software workaround is a last resort.

If you want, I can write a userspace utility to package with your existing
tools that can be installed and launched from init to provide this
optimization feature to the 8131 PCI bridge that your card resides on, to
ensure that your card gets this necessary optimization.
Or, you can easily put this capability into your existing device driver.

I would just rather not assume too much when dealing with something that
can potentially have a large reprocussion.

On Mon, 30 Aug 2004, Alan Cox wrote:

> On Llu, 2004-08-30 at 15:02, Richard B. Johnson wrote:
> > Hello all,
> >
> > The Celistica server with the AMD chip-set has very poor
> > PCI performance with Linux (and probably W$ too).
> >
> > The problem was traced to incorrect bridge configuration
> > in the HyperTransport(tm) chips that connect up pairs
> > of slots.
> Can you get Celestica to mail me their PCI subvendor
> id/devid's for the problem configuration or DMI strings
> and then we can do a PCI quirk properly for this.
> Alan
> -
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Arthur Perry
Linux Systems/Software Architect
Lead Linux Engineer
CSU Validation Group
Celestica, Salem, NH

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