[Beowulf] Cheap SDR IB

Gilad Shainer Shainer at mellanox.com
Thu Jan 31 09:54:10 PST 2008


Donnerstag, 31. Januar 2008, meintest Du:

	
	

	SA> On Jan 30, 2008, at 6:20 PM, Gilad Shainer wrote:

	
	

	>> For BW, Lx provides ~1400MB/s, EX is ~1500MB/s and ConnectX
is  

	>> ~1900MB/s

	>> uni-directional on PCIe Gen2.

	
	

	>> Feel free to contact me directly for more info.

	
	

	>> Gilad.

	
	

	SA> My god, IB bandwidths always confuse me. :-)

	
	

	SA> I thought IB SDR was 10 GB/s signal rate and 8 Gb/s data
rate. How do

	SA> you squeeze ~1400 MB/s out of 8 Gb/s?

	
	

	The 1400 MB/ are probably for DDR mode which is 20 GB/s signal
rate and 16 GB/s data rate. 

	
	

	SA> I see you offer Lx cards in PCIe 4x and 8x. Again, PCIe is
encoded at

	SA> 10bit/8bit so the data rate is 8 Gb/s. So the above value is
for your

	SA> 8x cards only, no? The thread is about your 4x cards, no?

	
	

	Values are for x8 cards. 

	
	

	I tested the x4 cards a few months ago and what i measured was: 

	
	

	- Latency was not affected by the slower PCIe connection

	- Bandwidth dropped to 700-750 MB/s where it is 900 to 950 with
a x8 SDR card. 

	 

	Gilad: Lx is been used in PCIe x8 and x4, and in the PCIe x8
adapters - you can find IB SDR and IB DDR. The cards mentioned in the
article are the IB SDR PCIe x4 with provide the same low latency, and
the BW is limited by the PCIe to ~750MB/s. With the recent chipsets it
is little bit higher.
	 
	 
	 

	
	

	(Intel MPI Benchmarks, pingpong)

	
	

	It depends on your application if the lower bandwidth affects
performance.

	
	

	Regards,

	Jan

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