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

On 06/18/2012 06:27 PM, Stephen Hemminger wrote:

On Mon, 18 Jun 2012 17:52:43 -0700
greearb@candelatech.com wrote:

quoted

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.

Signed-off-by: Daniel Baluta<redacted>
Signed-off-by: Ben Greear<redacted>

What is the justification (other than standard) for making this
tunable. Why would you want to do this? Why shouldn't the stack be adjusting
it for you (based on other heuristics)? Or is this just for testing interoperation
with TCP stacks that have wonky ACK policies. There are already too many TCP tunable
parameters for general usage.

tcp over wifi performance sucks, and tuning it to delay acks by 10 or 20 segments
gives a decent performance boost.

It is beyond me to write something that auto-tunes this, but even if someone
did, it's virtually guaranteed that someone somewhere will get better results
by tuning their application directly.

I honestly didn't even read the RFC section in question..just stole the
description text from the original patch by Daniel Baluta.

Thanks,
Ben


-- 
Ben Greear [off-list ref]
Candela Technologies Inc  http://www.candelatech.com