Re: [8023-CMSG] Purpose
All,
Doesn't preemption just introduce the same kinds of complexities at the
MAC as SAR? As soon as you start breaking frames apart, you need to have
special buffers and state machines for re-assembling them at the
receiver. With 8 priorities, preemption could get stacked up to 8 deep
and the receiver will need to sort all that out. It seems like a lot of
trouble to go to to save <1 uS per hop (on average) when queuing
latencies can easily reach 1 to 3 orders of magnitude more.
Gary
-----Original Message-----
From: owner-stds-802-3-cm@listserv.ieee.org
[mailto:owner-stds-802-3-cm@listserv.ieee.org] On Behalf Of Arthur
Marris
Sent: Wednesday, May 05, 2004 1:33 AM
To: STDS-802-3-CM@listserv.ieee.org
Subject: Re: [8023-CMSG] Purpose
Tom,
Preemption can be specified in such a way that the preempted frames
are not dropped. The transmission of the of the preempted frames would
be suspended to allow higher priority frames to pass by and then resume
without resending the previously sent frame data.
This could be done by specifying two (or more) channels in the MAC
for different priority levels. The higher priority channel could preempt
the lower priority channel. The priority level of the frame being
transmitted and the preemption control would be communicated between
MACs through the PHY using different codes for start of packet and end
of packet. Using this mechanism means there would be no need for the
MACs to examine the innards of the frame to discover the frame's
priority.
Arthur.
-----Original Message-----
From: owner-stds-802-3-cm@listserv.ieee.org
[mailto:owner-stds-802-3-cm@listserv.ieee.org] On Behalf Of Thomas
Dineen
Sent: Tuesday, May 04, 2004 7:57 PM
To: STDS-802-3-CM@listserv.ieee.org
Subject: 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.
>>>
>>>
>>>
>>>
>>
>