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Comment by gbrown_ | Hacker News Reader

    Currently the most powerful weather forecasting computer is the UK Met Office
    machine, a Cray XC40 machine that delivers 6.8 petaflops and sits at number 11
    on the TOP500.

But this does not run the production forecast, that is done on one of two smaller machines which are capable just over 3 petaflops.

Source: I work for Cray at the UK Met Office.

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So the XC40 is used for research purposes? I'd be very interested to know more about your workflow.

That XC40 system is for collaboration work but it also serves as a test bed of sorts as it's so much lager users can try scaling up models on it.

What is the 'bigger' one used for then?

Edit : Answered while I was posting this

This is a very good point. Large clusters are typically shared resources, this is a big problem for the US forecasts since we have to balance the time/cpu and timeliness of results.

Well production runs on one of two identical machines. This kind of redundancy is not uncommon for weather sites. One of these will run the production version of the forecast and the other will be running version n+1. We flip between the two semi-regularly to ensure if one machine were to have a major issue production would be unaffected if forced to move to the other machine. Smaller weather sites may have a single machine partitioned into two.

Please can you tell us what forecasts it is running then?

What does it do then?

Is it running fortran? Which version?

Not sure exactly what you're after here. The systems have assorted versions of Fortran compilers from both Cray and Intel. With regards to versions of the language standard used/ required for the models I'm not sure.

It runs Cray Linux (based on SUSE) and assuredly somebody is running Fortran on it in some form.

Many people think these large machines are intended for a small handful of users that run extremely large jobs. That's typically not the case; they typically have tens of users per day up to hundreds per week, depending on institution. With diverse users comes diverse software and the challenges based on that.

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