Re: [8023-CMSG] Purpose
Arthur,
I agree that preemption is a fine idea, but in my view it falls into the
"not worth the effort" category. Assuming that any new definition that
we could make will not be standardized until 2006 & will be commonly
available in silicon at least a year later, I think we can safely ignore
any Ethernet interfaces below 1Gbps. Even Gigabit Ethernet seems
somewhat pedestrian for high-end data center applications and I would
suggest that anyone concerned about the latency penalty of the frame in
progress at Gigabit speed would be well advised to migrate to 10G before
2007.
In that timeframe, a user will have the choice of 10GBASE-CX4 and
10GBASE-T for (cheap) copper interfaces. The former seems ideal for data
center as it is extremely low latency and targeted at the shorter
distances necessary for system-system communication. If the distances
involved force a requirement of distances up to 100m, making 10GBASE-T a
necessity, then the latency budget will be swamped by the physical
distance (500ns @ 100m) and the PMA/PCS latency of 10GBASE-T (probably
~1uS).
A maximum length frame in progress at 10Gbps will take ~1.2uS, making
the average gain due to pre-emption ~600uS (ignoring packet mix and link
utilization). Even taking the maximum delay (which will map to the delay
jitter component), the order of magnitude is similar to the fixed delay
of 10GBASE-T and therefore cannot possibly lead to a significant
reduction for systems using that technology.
Assuming that the speed-crazed implementor chooses 10GBASE-CX4 and
wishes to eliminate the 1.2uS max jitter then there are two options. The
first is preemption - which can significantly reduce this (depending on
the definition) but will involve significant new work. The alternative
is to reduce the MTU - which involves no new work. Changing the MTU from
1500 bytes to 500 bytes reduces the maximum jitter to 400nS at the
expense of ~3% extra overhead. Further reductions can be achieved for
larger overheads - which is a tradeoff that can be made at system
configuration time. I'm fairly sure that some will argue that the MTU
needs to be increased (to 9k, 16k, 64k or higher) because
software/firmware based NICs cannot encapsulate small frames at line
speed and 1982 vintage routers cannot switch line rate streams of
minimum size packets. I would suggest that anyone who is serious enough
to be asking for a new standard to improve latency should be using
hardware acceleration for packetization and true wire speed switch fabrics.
Assuming that the MTU has been reduced to 400nS, smart switch fabric
designers might wish to employ some techniques which can reduce the
jitter further at the expense of an increase in fixed latency. Given
that the fixed latency of the copper interconnect is approaching the
same magnitude, this seems like a reasonable tradeoff to make for system
performance (assuming that delay variation is the problem).
In summary, the net gain that can be achieved by preemption is too small
to make a difference except in the most extreme circumstances. For most
applications, current standards can be utilized (at layer 1 & 2) to
attain acceptable performance therefore the demand for silicon
implementing a new standard will be limited to a niche of a niche. If
the application area is sufficiently small then more exotic (or
targeted) technologies may have a competitive edge - there will be no
"Ethernet advantage."
Hugh.
Arthur Marris wrote:
>Jonathan,
> The presentation you gave in March at the Data Center Ethernet CFI
>suggested preemption as an area for exploration.
>
> Preemption would require a minor change to the PCS to support extra
>control-codes.
>
> Supporting preemption seems like a worthwhile objective as every
>microsecond is precious in cluster computing.
>
>Arthur.
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