Re: [RFC] TCP: Support configurable delayed-ack parameters.

On Mon, 2012-06-18 at 17:52 -0700, greearb@candelatech.com wrote:

From: Ben Greear <redacted>

RFC2581 ($4.2) specifies when an ACK should be generated as follows:

" .. an ACK SHOULD be generated for at least every second
  full-sized segment, and MUST be generated within 500 ms
  of the arrival of the first unacknowledged packet.
"

We export the number of segments and the timeout limits
specified above, so that a user can tune them according
to their needs.

Specifically:
	* /proc/sys/net/ipv4/tcp_default_delack_segs, represents
	the threshold for the number of segments.
	* /proc/sys/net/ipv4/tcp_default_delack_min, specifies
	the minimum timeout value
	* /proc/sys/net/ipv4/tcp_default_delack_max, specifies
	the maximum timeout value.

In addition, new TCP socket options are added to allow
per-socket configuration:

TCP_DELACK_SEGS
TCP_DELACK_MIN
TCP_DELACK_MAX

In order to keep a multiply out of the hot path, the segs * mss
computation is recalculated and cached whenever segs or mss changes.

I know David was worried about this multiply, but current cpus do a
multiply in at most 3 cycles.

Addding an u32 field in socket structure adds 1/16 of a cache line, and
adds more penalty.

Avoiding to build/send an ACK packet can save us so many cpu cycles that
the multiply is pure noise.