Re: [RFC PATCH v2 0/9] bql: Byte Queue Limits

On Sun, 7 Aug 2011 21:43:13 -0700 (PDT)
Tom Herbert [off-list ref] wrote:

Changes from last version:
- Simplified and generalized driver interface.  Drivers need to
  implement two functions:
    netdev_tx_completed_queue: Called at end of transmit completion
      to inform stack of number of bytes and packets processed.
    netdev_tx_sent_queue: Called to inform stack when packets are
      queued.

    netdev_tx_reset_queue: is optional to reset state in the stack

- Added new per queue flags that allow stack to stop a queue
  separately from driver doing this.  Driver continue using the
  same functions to stop queues, but there are two functions that
  the stack calls (to check if queue has been stopped by driver or
  stack:

  netif_xmit_stopped,netif_xmit_frozen_or_stopped

- Added example support for bnx2x and sfc (demonstrates operation over
  multi-queue)

- Removed BQL being under CONFIG_RPS (didn't add CONFIG_BQL)

- Still needs some more testing, including ishowing benfits to high
  priority packets in QoS.
----

This patch series implements byte queue limits (bql) for NIC TX queues.

Byte queue limits are a mechanism to limit the size of the transmit
hardware queue on a NIC by number of bytes. The goal of these byte
limits is too reduce latency caused by excessive queuing in hardware
without sacrificing throughput.

Hardware queuing limits are typically specified in terms of a number
hardware descriptors, each of which has a variable size. The variability
of the size of individual queued items can have a very wide range. For
instance with the e1000 NIC the size could range from 64 bytes to 4K
(with TSO enabled). This variability makes it next to impossible to
choose a single queue limit that prevents starvation and provides lowest
possible latency.

The objective of byte queue limits is to set the limit to be the
minimum needed to prevent starvation between successive transmissions to
the hardware. The latency between two transmissions can be variable in a
system. It is dependent on interrupt frequency, NAPI polling latencies,
scheduling of the queuing discipline, lock contention, etc. Therefore we
propose that byte queue limits should be dynamic and change in
iaccordance with networking stack latencies a system encounters.

A couple of other related points:

1. Need a way to configure the values.  My preference would be new
   netlink attribute and via 'ip link set' ...

2. Does this need to know the speed of the link (autonegotiation) or
   is it self clocking?