[openib-general] ip over ib throughtput

[Stephen Poole]

Have you done any "load" analysis of a 2K .vs. 4K MTU ? Your analogy 
of having 2G as a total message size is potentially flawed. You seem 
to assume that 2G is the end-all in size, it is not. What about when 
you want to (down the road) use IB for files in the 1-10TB in size. 
Granted, we can live with 2G, but it is not some nirvana number. 
Second, with the 2G limit on messages sizes, only determines the 
upper bound in overall size, I could send 2G @ 32bytes MTU. So, the 
question is, how much less of a system load/impact would a 4K MTU be 
over a 2K MTU. Remember, even Ethernet finally decided to go to Jumbo 
Frames, why, system impact and more. Remember HIPPI/GSN, the MTU was 
64K, reason, system impact. The numbers I have seen running IPoIB 
really impact the system.

Steve...

At 10:38 AM -0800 1/5/05, Diego Crupnicoff wrote:
>Note however that the relevant IB limit is the max ***message 
>size*** which happens to be equal to the ***IB*** MTU for the 
>current IPoIB (that runs on top of IB UD transport service where IB 
>messages are limited to a single packet).
>
>A connected mode IPoIB (that runs on top of IB RC/UC transport 
>service) would allow IB messages up to 2GB long. That will allow for 
>much larger (effectively as large as you may ever dream of) ***IP*** 
>MTUs, regardless of the underlying IB MTU.
>
>Diego
>
>  > -----Original Message-----
>>  From: Hal Rosenstock [<mailto:halr at voltaire.com>mailto:halr at voltaire.com]
>>  Sent: Wednesday, January 05, 2005 2:21 PM
>>  To: Peter Buckingham
>>  Cc: openib-general at openib.org
>>  Subject: Re: [openib-general] ip over ib throughtput
>>
>>
>>  On Wed, 2005-01-05 at 12:23, Peter Buckingham wrote:
>>  > stupid question: why are we limited to a 2K MTU for IPoIB?
>>
>>  The IB max MTU is 4K. The current HCAs support a max MTU of 2K.
>>
>>  -- Hal
>>
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-- 
Steve Poole (spoole at lanl.gov) 
	Office: 505.665.9662
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