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Date: Thu, 28 Apr 2011 03:29:46 -0400
From: Adam Stylinski <kungfujesus06@gmail.com>
To: freebsd-net@freebsd.org
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Subject: em0 performance subpar
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Hello,

I have an intel gigabit network adapter (the 1000 GT w/chipset 82541PI) whi=
ch performs poorly in Freebsd compared to the same card in Linux.  I've tri=
ed this card in two different freebsd boxes and for whatever reason I get p=
oor transmit performance.  I've done all of the tweaking specified in just =
about every guide out there (the usual TCP window scaling, larger nmbcluste=
rs, delayed acks, etc) and still I get only around 600mbps.  I'm using jumb=
o frames, with an MTU of 9000.  I'm testing this with iperf.  While I reali=
ze that this may not be the most realistic test, linux hosts with the same =
card can achieve 995Mbit/s to another host running this.  When the Freebsd =
box is the server, Linux hosts can transmit to it at around 800 something M=
bit/s.  I've increased the transmit descriptors as specified in the if_em m=
an page, and while that gave me 20 or 30 more mbit/s, my transmit performan=
ce is still below normal. =20

sysctl stats report that the card is trigger a lot of tx_desc_fail2:
	dev.em.0.tx_desc_fail2: 3431

Looking at a comment in the source code this indicates that the card was no=
t able to obtain enough transmit descriptors (but I've given the card the m=
aximum of 4096 in my loader.conf tunable).  Is this a bug or a performance =
regression of some kind?  Does anybody have a fix for this?  I tried anothe=
r card with the same chip in a different box on 8-STABLE to no avail (the b=
ox I'm trying to improve performance on is on 8.2-RELEASE-p1).

Anybody manage to make this card push above 600mbps in ideal network benchm=
arks?  Any help would be gladly appreciated.
--=20
Adam Stylinski
PGP Key: http://pohl.ececs.uc.edu/~adam/publickey.pub
Blog: http://technicallyliving.blogspot.com

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