RE: [RFC PATCH] net: limit maximum number of packets to mark with xmit_more

From: Jakub Kicinski

Sent: 25 August 2017 20:34

On Fri, 25 Aug 2017 08:24:49 -0700, Jacob Keller wrote:

quoted

Under some circumstances, such as with many stacked devices, it is
possible that dev_hard_start_xmit will bundle many packets together, and
mark them all with xmit_more.

Excuse my ignorance but what are those stacked devices?  Could they
perhaps be fixed somehow?  My intuition was that long xmit_more
sequences can only happen if NIC and/or BQL are back pressuring, and
therefore we shouldn't be seeing a long xmit_more "train" arriving at
an empty device ring...

I also suspect that the packets could be coming from multiple sources.
So getting the sources to limit the number of packets with XMIT_MORE
set won't really solve any problem.

At some point the driver for the physical device will have to give it
a kick to start the transmits.

On the systems I've got (desktop x86) PCIe writes aren't really very
expensive.
Reads are a different matter entirely (2us into our fpga target).

	David.