[Beowulf] InfiniBand channel bundling?

[Prentice Bisbal]

Gilad,

If end-to-end is lower for FDR, then what latency is being measured for 
FDR that is higher than for QDR? According to Wikipedia, and the 
Mellanox website, FDR does have a better latency than QDR (0.7 
microseconds vs. 1.3 microseconds), but I and others on this list have 
heard that FDR has worse latency than QDR. Have we been misinformed, or 
does it depend on how you measure or define latency?

http://en.wikipedia.org/wiki/InfiniBand
http://www.mellanox.com/page/performance_infiniband

Prentice


On 10/29/2014 06:46 PM, Gilad Shainer wrote:
> End-to-end FDR latency is lower than end-to-end QDR latency - per published measurments that can be found in multiple places.
>
> Gilad
>
> -----Original Message-----
> From: Beowulf [mailto:beowulf-bounces at beowulf.org] On Behalf Of Jörg Saßmannshausen
> Sent: Wednesday, October 29, 2014 3:45 PM
> To: Beowulf Mailinglist
> Subject: Re: [Beowulf] InfiniBand channel bundling?
>
> Hi all,
>
> thanks again for the wealth of information.
>
> Now, given that I am not interested in transporting files over the IB network but I am doing parallel calculations, I would have thought that the latency here is more important than the speed?
> Thus, if FDR has a higher latency than QDR, does that mean my performance is decreasing when I am running a calculation between nodes?
>
> For those of you who are into Chemistry code: I am using VASP, cp2k, quantum espresso and cpmd mainly. All of that is plain wave code.
>
> All the best from a wet London
>
> Jörg
>
>
> On Mittwoch 29 Oktober 2014 Prentice Bisbal wrote:
>> On 10/28/2014 04:43 PM, Mark Hahn wrote:
>>> On Tue, 28 Oct 2014, John Hearns wrote:
>>>> Here is a very good post from Glenn Lockwood regarding FDR versus
>>>> dual-rail QDR:
>>>>
>>>> http://glennklockwood.blogspot.co.uk/2013/05/fdr-infiniband-vs-dual
>>>> -rail
>>>> -qdr.html
>>> indeed, very nice.  though also quite surprising - is it known that
>>> FDR is so terrible for latency?  seems astonishing to me.
>> Yes, it was known to me. I had already known that FDR was worse than
>> QDR for latency, but I don't remember my source. I don't know if I'd
>> characterize it as "so terrible", though.
>
> --
> *************************************************************
> Dr. Jörg Saßmannshausen, MRSC
> University College London
> Department of Chemistry
> Gordon Street
> London
> WC1H 0AJ
>
> email: j.sassmannshausen at ucl.ac.uk
> web: http://sassy.formativ.net
>
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-- 
Prentice Bisbal
Manager of Information Technology
Rutgers Discovery Informatics Institute (RDI2)
Rutgers University
http://rdi2.rutgers.edu