Mattyhawk
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Joined: Tue Nov 20, 2018 8:28 pm

RPi CLuster

Thu Dec 20, 2018 7:22 am

If I was making a cluster with two Rpi 3b+ s, I'm guessing the ram would become two GB or would they both work separately and hold the same information as each other?

In addition, the clock speed on the 3b+ is 1.4ghz so would the theoretical clock speed be 2.8ghz as there are two CPUs working at 1.4ghz?

My final question is what would be faster, a two stack zero w or a single 3b+

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rpdom
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Location: Chelmsford, Essex, UK

Re: RPi CLuster

Thu Dec 20, 2018 7:34 am

Mattyhawk wrote:
Thu Dec 20, 2018 7:22 am
In addition, the clock speed on the 3b+ is 1.4ghz so would the theoretical clock speed be 2.8ghz as there are two CPUs working at 1.4ghz?
There are four ARM CPU cores in the 3B+, but the clock speed will still be 1.4GHz max. Just it will be able to do four things at 1.4MHz each.
what would be faster, a two stack zero w or a single 3b+
The 3B+. Each of the four ARMv8s in it are many times more powerful than the single ARMv6 in the zero. Also, the zero runs at 1GHZ and has half the memory. Not that it isn't a great little machine for many purposes. I have a couple I run all the time.

The efficiency of clustering depends on how you use it. For a cluster of Pi (Bramble is one that has been done), distributed processing is best, where you can break down a big task into many independant sub tasks and run each on one system. Processing an image could be broken down into areas. Calculating a large number is not so easy.

One example of distributed processing is the [email protected]/BOINC project, where data is grabbed from a radio telescope, broken up into chunks and sent out to people to process and analyse. The total amount of data was huge and it would take ages to process on a single powerful machine, but by breaking it up they could harness the power of millions of systems.

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DougieLawson
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Re: RPi CLuster

Thu Dec 20, 2018 2:43 pm

Mattyhawk wrote:
Thu Dec 20, 2018 7:22 am
If I was making a cluster with two Rpi 3b+ s, I'm guessing the ram would become two GB or would they both work separately and hold the same information as each other?
Loosely coupled clustering does not share RAM. Each Raspberry would still have the same OS and services running before you run your clustered application.

For clustering to be effective (whether the cluster is loosely or tightly coupled) you need to break your large workload into small atomic chunks that can be processed on each of the cores of the cluster (eight chunks in your case). You then have a process to aggregate the results from each chunk into a single results set.

What you're asking for is analogous to connecting your laptop to your partners and expecting the dual laptop to run your favourite game twice as fast as running it on one laptop. Loosley coupled parallel computing has a need for a very specialised workload with data splitting and aggregating services to feed the cluster.

You will also suffer from the USB/Ethernet bottleneck as all of the data in and out of a Raspberry is either coming in through the network or from a USB device (or, more likely, in on the network, to be temporarily held on a USB device). That is limited to 480Mb/s because it ALL runs though the same SMSC9514 hub chip.

With tightly coupled parallel computers you'll have multiple paths to the disks, you'll have extremely high speed links to shared memory (which may be running on it's own specialised processor) and a very high speed connection to the network. That's not possible with a Raspberry.
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