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RE: [10GBASE-T] latency


This is a tough question because latency is important for some applications that might use RDMA NICs but there are also constraints on the power available. An RDMA NIC is an interface card that includes the RDMA protocol plus a TCP/IP offload engine and MAC/PHY. The MAC/PHY would usually be Ethernet). 

This is kind of long so here is the executive summary:
Practically all of the the 10GBASE-T market will require reasonable power requirements. There is additional market available if the PHY is very low latency. However BMP is very dependent on resonable power so if a trade-off of power for latency pushes the power too high, one will lose more market than one gains. Note that low latency in Infiniband and Fibre Channel can be around 100 ns port to port through a switch. 

--- The details ---

The upper layers on these cards use a plenty of power themselves so I doubt there is more than 5 W available for the 10GBASE-T PHY given the power that a card slot can provide. That number would probably be workable though painful. Less would be better. Much more and it will probably be hard to find slots that can provide the power and remove the heat. There may be early bleeding edge products made with higher power but for broader use the technology should be able to get here.

Hopefully other NIC vendors will chime in if they disagree about the power.

Like Ethernet NICs, RDMA NICs are intended to support a wide variety of applications. Some of these applications are pretty traditional networking applications and aren't especially latency sensitive. Other potential applications such as storage and clustering are currently served by more specialized networks (e.g. Fibre Channel and the proprietary predecessors to Infiniband) and are latency sensitive.

What do clustering (Infiniband) and storage (Fibre Channel) customers consider low latency?
In Infiniband, the systems vendors generally wanted less than 100 ns port to port through the switch. Fibre Channel switches are about the same. In both technologies they typically are using cut through switching to get this speed. Ethernet switches moved away from their early cut-through operation and generally have much higher latency. If Ethernet wants to serve the very latency sensitive applications, then more than PHYs has to be low latency.

Neither of these technologies are planning on a 10GBASE-T type PHY. They have PHYs similar to CX4 and the optical PHYs. Infiniband is working on a quad speed version of their existing 2.5 Gig signaling (as 802.3 may end up doing if the backplane study group is chartered). It could be argued that for these very latency applications, Ethernet also can use the CX4 and optical PHYs. 

I'm not sure what the latency range of the proposals under consideration currently is. It seems likely that even the fastest of them doesn't achieve the ultra low latency that the systems vendors want for this class of application.

Given this, it makes sense to accept some extra delay in return for lower power.


-----Original Message-----
From: Stephen Bates []
Sent: Friday, February 20, 2004 12:34 PM
To: THALER,PAT (A-Roseville,ex1)
Subject: RE: [10GBASE-T] latency

Hi All

My thanks to everyone who has responded to my email. 

The responses I've been getting tend to suggest that PAUSE should not be
(and is not) enabled in most Ethernet systems. If flow control is
required it should be handled higher up the stack. This obviously
increases the latency and if TCP/IP is implemented in software no exact
bound on latency can be given since it will be architecture specific.

However, although latency is not an issue for PAUSE it is a major issue
for certain applications 10G may be targetting. I believe this brings us
back to Brad's original request for some figures on end to end latency
for applications such as cluster computing and RDMA.

Serag mentioned that we should stick to the low latency solutions if the
power remains comparable with that of 10G optical transponder. My
concern is that the balance between digital and analog power will be
totally biased to the analog. This implies power consumption will not
drop as much with technology scaling. In this case we will not see the
same kind of power consumption drop over time that we saw for




Dr. Stephen Bates

Dept. of Electrical and Computer Engineering      Phone: +1 780 492 2691
The University of Alberta                         Fax:   +1 780 492 1811
Canada, T6G 2V4