Large Send Offload

NTDEV
1

All,

I have a NIC that supports LSO but when I enable the feature the transfers
are actaully a LOT slower. I am testing on Windows 2003 SP1 and SP2 – same
results. Windows Firewall is turned off. IP packet filtering is also turned
off.

Is anyone else experiencing this?

TIA,
JJ

2

I can think of only two things:

1.) Your large send waits until all derived packets are actually sent before
completing. Better to transfer entire large send packet to HW in one
operation, then complete the large send while derived segment packets were
being sent.
2.) The large send operation actually loads your hardware “atomically” with
many more packets then in normal operation. For example, I doubt that you
actually see more than a few normal packets on any single call to
NdisSendPackets, but you load the HW with 20 (or more…) derived segment
packets during large send. Your HW prioritization might be such that
receives are not indicated at all while the HW transmitter is busy. This
could have an indirect influence on performance.

Just food for thought.

Good luck,

Thomas F. Divine

-----Original Message-----
From: xxxxx@lists.osr.com [mailto:bounce-303282-
xxxxx@lists.osr.com] On Behalf Of jjjames1
Sent: Monday, October 15, 2007 7:07 PM
To: Windows System Software Devs Interest List
Subject: [ntdev] Large Send Offload

All,

I have a NIC that supports LSO but when I enable the feature the
transfers
are actaully a LOT slower. I am testing on Windows 2003 SP1 and SP2 –
same
results. Windows Firewall is turned off. IP packet filtering is also
turned
off.

Is anyone else experiencing this?

TIA,
JJ

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3

Is this 1G Ethernet? 10G? What does a “LOT slower”
mean? What is your interpacket gap with and without
LSO? Is your LSO function correct (are the fields of
the IP and TCP header being plugged correct)?

I do not see this - with LSO, performance is better on
Windows Server 2003 SP1 and SP2.

— jjjames1 wrote:

> All,
>
> I have a NIC that supports LSO but when I enable the
> feature the transfers
> are actaully a LOT slower. I am testing on Windows
> 2003 SP1 and SP2 – same
> results. Windows Firewall is turned off. IP packet
> filtering is also turned
> off.
>
> Is anyone else experiencing this?
>
> TIA,
> JJ
>
>
>
> —
> NTDEV is sponsored by OSR
>
> For our schedule of WDF, WDM, debugging and other
> seminars visit:
> http://www.osr.com/seminars
>
> To unsubscribe, visit the List Server section of OSR
> Online at
> http://www.osronline.com/page.cfm?name=ListServer
>

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4

Are you telling ndis how much data you have been sending in each LSO packet?
This is a common mistake to do in such cases.

// Tell NDIS how much we have sent.
PktExt->NdisPacketInfo[TcpLargeSendPacketInfo] = UlongToPtr(PacketLength);

you can also try to change the maximum size of LSO that you report to NDIS.

And if those two don’t help, try looking at the network on both sides and see if there are packets retransmitted.