In short, he wasn't wrong. His arguments were correct, but a combination of happenstance, intransigence, and a vast infusion of research dollars kept Wintel dominant on the desktop, while the world moved away from it.
The micro- v. macro-kernel debate is moot; all of the interesting development is happening elsewhere in kernel-land. On that, he was wrong. But RISC will dominate CISC unless Intel manages to pull a miracle out of their ass, <s>and GNU will dominate, at least in terms of #installs, through Android.</s> nope, I stand corrected.
So Linus was right about what mattered in the 90's, and Tanenbaum had his finger on the pulse of history. Both knew what they were talking about, and both were right in their own way.
Although reading the Usenet postings, Linus does come across as more of an arrogant upstart, and less of a dickish master than usual. That alone is worth the read.
He was "right" in that he made a prediction that did not come anywhere near true?
Sorry, he was wrong.
As to the 2nd point about x86 vs "RISC" processors, it turns out he was so massively wrong the very basis of his understanding was incorrect. CISC processors are dead today, they stopped being a substantial part of the market in the late 90s. I'm sure they still exist somewhere in new devices (RAD hardened Pentiums, perhaps) but for the most part the CISC vs RISC battle is over, and RISC won overwhelmingly. But Tannenbaum thought that a necessary consequence of that would be that the x86 architecture would die due to the weaknesses of CISC processor designs. But what actually happened is that Intel (and later AMD and others of course) started making processors with a RISC core that support the x86 (IA32) instruction set through transparent op-code translation. Every Intel cpu since the Pentium-Pro has worked that way (and every AMD CPU since the Athlon).
You can't just wave your hands and say "yeah, but he was fundamentally right in some ways though there were some things he couldn't have foreseen". That's part of the deal, there's always something that you can't foreseen. Imagining that CISC's weaknesses are identical to the weaknesses of the x86 architecture are just the sort of naivety and shallow reasoning that can lead you to make woefully wrong predictions.
Recently on a flight I sat next to a Ph.D. that ran a chip design consulting firm. In this fascinating discussion, he talked about the internal design of AMD and Intel processors and said exactly this. AMD and Intel haven't had hardware based x86 computation for a very long time.
Instead, they have a RISC style pipeline with an op-code translation layer that decodes instructions into smaller RISC-like instructions that get run through the pipeline.
So really, modern x86 processors are more like x86-compatible processors.
"CISC processors are dead today [...] Intel (and later AMD and others of course) started making processors with a RISC core that support the x86"
That's a pretty far fetched argument. RISC/CISC is about
instruction set, and the x86 instruction set is CISC.
Of course since the big RISC/CISC battle the implementations have converged a lot on the microarchitecture
level, primarily because the transistor budget sweet spot targeted by RISC melted away and the amount of chip area saved in instruction decode and ISA simplicity was later dwarfed by out-of-order machinery, caches etc.
So an equally valid argument (as "CISC is dead") is "RISC is dead" since RISC chips today have brainiac instructions and pipelines like divide/multiply, unaligned access, variable length instructions (Thumb on ARM), out-of-order execution etc.
That's sort of like saying that any RISC machine becomes a CISC machine at soon as you install the JVM on it (With the caveats that I don't know if the JVM has a CISC instruction set)
Right. I think what Tannenbaum overlooked was the massive transistor budgets that were on their way thanks to Moore's law. Where early CISC chips had to spend a substantial fraction of their transistors on instruction decoding, the absolute number of transistors required for that didn't increase nearly as fast as the total on the chip. So now we have quite complex instruction decoders that translate CISC to RISC, and they're still just a small fraction of the chip.
He was squarely wrong in that he made concrete predictions and these fell off dramatically.
He said in the early 90's that x86 would be dead in 5 years. If that's not a wrong prediction I don't know what is.
As for microkernels being superior to macrokernels, the trend has been to evolve into hybrid kernels (one of which Linux is now in practice). CISC vs RISC: same outcome. Hybrid approaches have come on top. Modern x86 processors are RISC inside, CISC outside and this produces concrete advantages by needing less memory bandwidth. Even if the instruction set was nominally RISC, we'd do more instruction pipelining producing a similar result. It's got to the point where the difference is nominal. We still call them x86 but they are fundamentally different processors. We still call them RISC and they do instruction compositing now.
He was WRONG with capitals, as usually "religious" and opinionated people are. There's rarely black and white in the real world. It's shades of grey.
Also, being off by a 400% in time scale, even if the outcome is similar as predicted, is being wrong no ifs or buts. Predictions like this mean total practical failure in any decision making.
GNU will dominate, at least in terms of #installs, through Android.
Android is not GNU, although its kernel is Linux. So much so that it is an example cited by GNU's GNU/Linux FAQ in order to make the case that Linux and GNU/Linux are different.
Yes, this is exactly right on both counts. He's not making an argument about a generic open source POSIX operating system being dominant, he's making a specific argument about Hurd, the GNU kernel. Which, incidentally, still hasn't been released.
"... Linus does come across as more of an arrogant upstart, and less of a dickish master than usual. ..."
There was a follow-up to this email joust detailed in "Just for Fun" [0] with this remark, "Maybe a year later, when Linus was in the Netherlands for his first public speech, he made his way to the university where Tanenbaum taught, hoping to get him to autograph Linus's copy of Operating Systems: Design & Implementation, the book that changed his life. He waited outside his door but Tanenbaum never emerged. The professor was out of town at the time, so they never met."
The book also detailed that the main reason for the spat was that Tanenbaum was publicly commenting. Hence the response.
The micro- v. macro-kernel debate is moot; all of the interesting development is happening elsewhere in kernel-land. On that, he was wrong. But RISC will dominate CISC unless Intel manages to pull a miracle out of their ass, <s>and GNU will dominate, at least in terms of #installs, through Android.</s> nope, I stand corrected.
So Linus was right about what mattered in the 90's, and Tanenbaum had his finger on the pulse of history. Both knew what they were talking about, and both were right in their own way.
Although reading the Usenet postings, Linus does come across as more of an arrogant upstart, and less of a dickish master than usual. That alone is worth the read.