[openib-general] [PATCH] ipoib tune rx ring size

[William Jordan]

On 6/20/05, Grant Grundler <iod00d at hp.com> wrote:
> On Mon, Jun 20, 2005 at 06:14:04PM +0300, Michael S. Tsirkin wrote:
> > Reducing the rx ring size gives me bandwidth win of about 1%.
> 
> Michael,
> Doesn't this sound counter-intuitive to you?
> 
> Can you share which configuration (chipset, CPU, IO bus, HCA)
> you measured this on?
> 
> I've also seen this to be true with gige - but only in one odd configuration.
> 
> thanks,
> grant

I'd be skeptical also. I've found this to be true in only one very
contrived instance.

When receiving UDP traffic on a slow host (no user level end to end
flow control), you can improve receive throughput by reducing the
number of outstanding receives. The CPU can be saturated receiving
packets which are overflowing the socket (being discarded), and the
user can be CPU starved (unable to empty the socket). In this case,
you are better off having the HCA miss the packets than you are having
the driver process each packet, only to discard it in the upper
layers.

This is not a real world situation, though.

-- 
Bill Jordan
SilverStorm Technologies