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40,000 Linux instances on a mainframe? Not quite.


I've written on this topic before but I keep running into the same misconception: that the IBM mainframe is a cost-effective platform for Linux because over 40,000 instances of Linux have been shown to run concurrently on a modern mainframe.

That demonstration was effectively a gimmick. No, I don't believe it was intended as a gimmick; only that it was widely misconstrued and, therefore, became indistinguishable from a gimmick in the media circus that resulted. It was depicted by the media as a practical demonstration that one could replace 40,000 PC's running Linux with a single mainframe. Alas, that was surely not the point of the demonstration.

Don't confuse a large number of 'logical' machines with physical ones. If a Pentium III had the ability in hardware to subdivide itself into thousands of functionally identical logical processors you would be able to run thousands of Linux instances on that one CPU. You probably see the problem that you would immediately encounter: each Linux instance would have only a tiny fraction of a percent of the PIII's processing power. Yes, you'll have thousands of distinct running instances of Linux, but they will be very slow when several of them try to do something cpu-intensive at the same time.

A mainframe CPU is not dramatically faster than (any other) microprocessor anymore. In recent years I've only been able to indirectly compare the benchmarks; it seems that IBM isn't interested in submitting it's mainframes for industry standard benchmarking these days. Bottom line: a 12-CPU mainframe is still a 12-CPU box, even if running 1,000 or 10,000 instances of Linux.

I am not making the claim that the demonstrated ability is useless; quite the contrary. There may be situations where the ability to isolate a large number of low-power Linux images from one another is sufficient to justify the cost of a mainframe. I don't expect such situations to be common, however.

In these days of Sun 10000's and other huge SMP Unix boxes, the mainframe is no longer a honker of a big computer. In fact, it has been a long time since even IBM's most powerful systems were traditional mainframes. Their own Unix systems long ago surpassed the mainframe in both raw performance and price/performance. Reliability, the ability to run existing OLTP and enterprise software, and manageability are the big reasons people still buy mainframes. And even those reasons diminish a bit with every passing year.

[Related: The Warmblooded Dinosaur: Linux on the Mainframe]
mail this link | score:7188 | -Ray, September 4, 2001 (Updated: April 18, 2007)
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