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

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2012-06-18 at 17:52 -0700, greearb@candelatech.com wrote:
> From: Ben Greear <greearb@candelatech.com>
> 
> 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.