[Beowulf] InfiniBand channel bundling?
brice.goglin at gmail.com
Thu Oct 30 08:08:17 PDT 2014
Do you want to compare FDR vs QDR when used in the exact same PCIe
slots? FDR often goes in Gen3 while QDR has been in Gen2 for a while.
According to the link below, Gen3 is responsible for pretty much all the
improvement between QDR/Gen2 and FDR/Gen3.
Le 30/10/2014 15:41, Prentice Bisbal a écrit :
> 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?
> 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.
>> -----Original Message-----
>> From: Beowulf [mailto:beowulf-bounces at beowulf.org] On Behalf Of Jörg
>> 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
>> 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:
>>>> 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
>> WC1H 0AJ
>> email: j.sassmannshausen at ucl.ac.uk
>> web: http://sassy.formativ.net
>> Please avoid sending me Word or PowerPoint attachments.
>> See http://www.gnu.org/philosophy/no-word-attachments.html
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