Amdahl's Law Revised for Hyper-Threading

In the book that Rich Gerber and I wrote on Intel's Hyper-Threading Technology, we present a revised version of Amdahl's Law that reflects the constraints of Hyper-Threading Technology. There was an error in that equation that was recently reported by Gang Chen of Shanghai Jiao Tong University. The corrected formula is:

Speedup = 1 / (S + (1-S)/(0.67n) + Hn)

where S = sequential time, n = number of logical processors, H = overhead.

Clay Breshears of Intel was kind enough to double-check the revision. Ignoring overhead, if a program is 99% parallelized on a Hyper-Threading chip with two logical processors, the speedup is:

Speedup = 1 / (.01 + .99 / (.67*2)) = 1 / 0.749 = 1.34

Intel has long suggested that a 30% overall speedup was the most improvement that could realistically be achieved. The 34% improvement shown here is more theoretical than real because no overhead is included and the code is 99% parallelized. So, Intel's projections are consistent with this formula.

Thanks to Clay and to Gang Chen.

A Really Useful Ruby Book

In my latest column in SD Times, I try to point out that not everything is super-cool about Ruby. One of the things I point to is the lack of good books. Sure the pick-axe book is a good place to start, but you quickly encounter the need for a practical reference that illustrates the idioms for doing standard things.

O'Reilly and Associates has just released such a book, Ruby Cookbook, (ISBN 0-596-52369-6), which contains more than 800 pages of helpful routines accompanied by thoughtful explanations. It shows how things are normally coded in Ruby. It's an impressive work especially because of its tremendous sweep: whether you need string handling, file I/O, database programming, network programming, multitasking, or accessing BitTorrent, it's all covered in this book. At a $33 street price, the book is a steal, and most Ruby developers are likely to keep a copy beside their workstation.

Other Ruby books are on the way. Most specialize on Rails, but some focus on the language as such. (Surely, there will be the bumper crop of "Ruby in 7 Days" kind of titles and other popularizing volumes, as well). So, my complaint about the lack of good books might be resolved by year end. We'll see. Meanwhile, this volume is a huge step forward.

Joel Spolsky on Writing Your Own Language

Here is a neat post about why Joel's outfit wrote their own language, as well as the benefits and drawbacks of this approach.

The new bytecode in Java 6 explained

For a long time, I've been looking for a cogent explanation of the new bytecode, called invokedynamic, that is being introduced in Java 6 (Mustang). This bytecode facilitates calls to scripting language methods in a context where data items are not strongly typed. Beyond that brief description, though, it's hard to find much info. Here, however, is a link that gives a good overview of the role of this bytecode and where things stand now regarding its implementation.