On 01.11.2012 00:50, Jack F Vogel wrote:
Author: jfv
Date: Wed Oct 31 23:50:36 2012
New Revision: 242421
URL: http://svn.freebsd.org/changeset/base/242421
Log:
A few important fixes:
- Testing TSO6 has led me to discover that HW RSC is
a problematic feature, it is ONLY designed to work
with IPv4 in the first place, and if IP forwarding
is done it can't be disabled as LRO in the stack,
also initial testing we've done at Intel shows an
equal performance using TSO[46] on the TX and LRO
on RX, if you ran older code on 82599 or later hardware
you actually could have detrimental performance for
this reason. So I am disabling the feature by default
and all our adapters will now use LRO instead.
Yes, it's very important that LRO is *not* used when forwarding
is enabled (= acting as a router).
- If you have flow control off and multiple queues it
was possible when the buffer of one queue becomes
full that all RX movement is stalled, to eliminate
this problem a feature bit is now set that will allow
packets to be dropped when full rather than stall.
Note, the default is to have flow control on, and this
keeps this from happening.
- Because of the recent fixes in the stack, LRO is now
auto-disabled when problematic, so I have decided to
enable it by default in the capabilities in the driver.
A very important cautionary note here: LRO is only good when combined
with very low RTTs (that is in LAN environments). On everything over
5ms is breaks the TCP ACK clock badly and performance will suffer
greatly. This is because every ACK increases the congestion window.
With a greatly reduced ACK rate the ramping up of CWND on startup and
after a loss event is severely limited. Combined with ABC (appropriate
byte counting) where the CWND increases only once per ACK by at most
one MSS the effect is greatly pronounced as well. The higher the RTT
goes the worse the effects become. I haven't checked yet whether our
soft-LRO does ACK compression or not. If it does, we need a workaround
and some tcp_input magic to reduce the negative impact. I'm looking
into it.
- There are some 1G modules used by some customers, a couple
small tweaks to properly support those in the media code.
- A note: we have now done some testing of TSO6 and using
LRO with IPv6 and it all works great!! Seeing line rate
in both directions in best cases. Thanks bz for your
excellent work!!
Indeed!
Modified:
head/sys/dev/ixgbe/ixgbe.c
--
Andre
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