Re: [8023-CMSG] Purpose
Gentle People:
An aspect of preemption that was not discussed below has just
come to mind. What would be the effect on both overall link utilization
and the low priority preempted flows?
First of all I assume that the preempted partial frames are just dropped
and thus must be retransmitted later. The entrenched 803.2 mind set prevents
any other viewpoint. As I see it this would in some cases reduce the
effective
link bandwidth for low priority flows by 50%. This would have a devastating
effect on overall link utilization if preemption were constantly occurring.
Next the low priority preempted flows would suffer greatly in a
preemption
scheme due to the constant drop and retransmission. This would in effect
be a
form of double discrimination, first they are low priority at queuing
and second
they are constantly being dropped and retransmitted.
Thomas Dineen
> Hugh Barrass wrote:
>
>> 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.
>>>
>>>
>>>
>>>
>>
>