latency (was: RE: cooling door)

Sun Mar 30 08:17:19 UTC 2008

On Sat, 29 Mar 2008, Frank Coluccio wrote:

> Understandably, some applications fall into a class that requires very-short
> distances for the reasons you cite, although I'm still not comfortable with the
> setup you've outlined. Why, for example, are you showing two Ethernet switches
> for the fiber option (which would naturally double the switch-induced latency),
> but only a single switch for the UTP option?

Yes, I am showing a case where you have switches in each rack so each rack 
is uplinked with a fiber to a central aggregation switch, as opposed to 
having a lot of UTP from the rack directly into the aggregation switch.

> Now, I'm comfortable in ceding this point. I should have made allowances for this
> type of exception in my introductory post, but didn't, as I also omitted mention
> of other considerations for the sake of brevity. For what it's worth, propagation
> over copper is faster propagation over fiber, as copper has a higher nominal
> velocity of propagation (NVP) rating than does fiber, but not significantly
> greater to cause the difference you've cited.

The 2/3 speed of light in fiber as opposed to propagation speed in copper 
was not in my mind.

> As an aside, the manner in which o-e-o and e-o-e conversions take place when
> transitioning from electronic to optical states, and back, affects latency
> differently across differing link assembly approaches used. In cases where 10Gbps

My opinion is that the major factors of added end-to-end latency in my 
example is that the packet has to be serialisted three times as opposed to 
once and there are three lookups instead of one. Lookups take time, 
putting the packet on the wire take time.

Back in the 10 megabit/s days, there were switches that did cut-through, 
ie if the output port was not being used the instant the packet came in, 
it could start to send out the packet on the outgoing port before it was 
completely taken in on the incoming port (when the header was received, 
the forwarding decision was taken and the equipment would start to send 
the packet out before it was completely received from the input port).

> By chance, is the "deserialization" you cited earlier, perhaps related to this
> inverse muxing process? If so, then that would explain the disconnect, and if it
> is so, then one shouldn't despair, because there is a direct path to avoiding this.

No, it's the store-and-forward architecture used in all modern equipment 
(that I know of). A packet has to be completely taken in over the wire 
into a buffer, a lookup has to be done as to where this packet should be 
put out, it needs to be sent over a bus or fabric, and then it has to be 
clocked out on the outgoing port from another buffer. This adds latency in 
each switch hop on the way.

As Adrian Chadd mentioned in the email sent after yours, this can of 
course be handled by modifying or creating new protocols that handle this 
fact. It's just that with what is available today, this is a problem. Each 
directory listing or file access takes a bit longer over NFS with added 
latency, and this reduces performance in current protocols.

Programmers who do client/server applications are starting to notice this 
and I know of companies that put latency-inducing applications in the 
development servers so that the programmer is exposed to the same 
conditions in the development environment as in the real world. This means 
for some that they have to write more advanced SQL queries to get 
everything done in a single query instead of asking multiple and changing 
the queries depending on what the first query result was.

Also, protocols such as SMB and NFS that use message blocks over TCP have 
to be abandonded and replaced with real streaming protocols and large 
window sizes. Xmodem wasn't a good idea back then, it's not a good idea 
now (even though the blocks now are larger than the 128 bytes of 20-30 
years ago).

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se