Re: [PATCH RFC 11/11] net/mlx5e: XDP TX xmit more

On 09/08/2016 11:16 AM, Tom Herbert wrote:

On Thu, Sep 8, 2016 at 10:19 AM, Jesper Dangaard Brouer
[off-list ref] wrote:

quoted

On Thu, 8 Sep 2016 09:26:03 -0700
Tom Herbert [off-list ref] wrote:

quoted

Shouldn't qdisc bulk size be based on the BQL limit? What is the
simple algorithm to apply to in-flight packets?

Maybe the algorithm is not so simple, and we likely also have to take
BQL bytes into account.

The reason for wanting packets-in-flight is because we are attacking a
transaction cost.  The tailptr/doorbell cost around 70ns.  (Based on
data in this patch desc, 4.9Mpps -> 7.5Mpps (1/4.90-1/7.5)*1000 =
70.74). The 10G wirespeed small packets budget is 67.2ns, this with
fixed overhead per packet of 70ns we can never reach 10G wirespeed.

But you should be able to do this with BQL and it is more accurate.
BQL tells how many bytes need to be sent and that can be used to
create a bulk of packets to send with one doorbell.

With small packets and the "default" ring size for this NIC/driver combination, is the BQL large enough that the ring fills before one hits the BQL?

rick jones