Subject: Hardware checksums slow down max throughput on ti(4) board?
To: None <>
From: Jeff Rizzo <>
List: current-users
Date: 12/17/2001 11:49:38
I wasn't sure if I should send this here or to port-i386, so forgive
me if it's too off-topic.

Using ttcp for some basic throughput testing, I find that without
hardware checksumming (ip4csum, etc) enabled, I can get a max
throughput of about 280Mbps on a TCP connection, and close to
800Mbps for raw UDP (with drops).  *With* hardware checksums enabled,
max throughput is about 227Mbps for a TCP connection, and 175Mbps for
raw UDP. (!)

(This is with the boards connected back-to-back, with no switch in between. 
 With a switch, performance is somewhat less than above)

I'm using a 1.5Y i386 kernel, with a 1.5.2 userland (though I replaced
the ifconfig binary, so I could use the {ip4,tcp4,udp4}csum options).

The GigE cards are 3com 3C985B-SX (64-bit cards, claim support for 33MHz
or 66MHz PCI)

The motherboards are Tyan Thunder LE S2510, with 64-bit PCI slots,
and jumper settings for either 33MHz or 66MHz.  I've currently got
it set for 66MHz PCI.

So, is this expected?  Perhaps I don't fully understand the purpose
of hardware checksumming...


Jeff Rizzo