tcp: Use segment size excluding any options for all cwnd calculations
ClosedPublic
Actions

Authored by rscheff on Nov 7 2024, 2:41 PM.

Details

Reviewers

tuexen
cc
glebius
thj
rrs

Group Reviewers

transport

Commits

rG22dcc8129385: tcp: Use segment size excluding tcp options for all cwnd calculations

Summary

Avoid sending small segments by making sure that cwnd is usually
calculated in full (data) segment sizes. Especially during loss
recovery and retransmission scenarios.

An alternative approach, and possibly more effective, could be to
ensure in tcp_output, that small segments are only sent, when ACKNOW
is set, or cwnd would allow sending the entire send buffer (but the
final segment being smaller than MSS).

Diff Detail

Repository

rG FreeBSD src repository

Lint

Lint Passed

Unit

No Test Coverage

Build Status

Buildable 60434
Build 57318: arc lint + arc unit

Event Timeline

rscheff created this revision.Nov 7 2024, 2:41 PM

Herald added a reviewer: transport. · View Herald TranscriptNov 7 2024, 2:41 PM

Herald added subscribers: melifaro, imp. · View Herald Transcript

rscheff requested review of this revision.Nov 7 2024, 2:41 PM

Harbormaster completed remote builds in B60434: Diff 146137.Nov 7 2024, 2:41 PM

tuexen accepted this revision.Nov 14 2024, 3:24 PM

This revision is now accepted and ready to land.Nov 14 2024, 3:24 PM

cc added inline comments.Nov 14 2024, 4:09 PM

sys/netinet/cc/cc_newreno.c
218	This is fast path and re-calculate MSS for every input draws my efficiency concern. I remember Lawrence once discussed about this, if I remember correctly. Why not cache the correct MSS in `tcpcb`?

Well, I had the same thought - the full MSS (including options) is less frequently used, that the mss without options...

So, it would certainly be more efficient to store the MSS exkluding options in tcpcb, and calculating the MSS inkl. option space only where that is needed...

But that should a different Diff IMHO, as this one is more in the break/fix category.

In D47474#1084985, @rscheff wrote:

Well, I had the same thought - the full MSS (including options) is less frequently used, that the mss without options...

So, it would certainly be more efficient to store the MSS exkluding options in tcpcb, and calculating the MSS inkl. option space only where that is needed...

But that should a different Diff IMHO, as this one is more in the break/fix category.

I want to test this patch to see if any segmentation being improved during loss recovery, as I found in https://wiki.freebsd.org/chengcui/testD43470.

Closed by commit rG22dcc8129385: tcp: Use segment size excluding tcp options for all cwnd calculations (authored by rscheff). · Explain WhyNov 14 2024, 6:38 PM

This revision was automatically updated to reflect the committed changes.

rscheff added a commit: rG22dcc8129385: tcp: Use segment size excluding tcp options for all cwnd calculations.

update:
Looks this patch has some significant reduction on fragment (data_size % MSS) > 0 out of TSO data chunks: testD47474
TSO not enabled: