On the Accuracy of Exponentials and Expositions

The following is a slightly edited version of my response to a Discussion on the Linkedin CPPE group, which is accessible to Members Only. It's written in the style of a journal reviewer. The original Discussion topic was based on a link to a blog-post. I've been asked to make my Linkedin review more widely available so, here tiz...

The blog-post Capacity Planning on a Cocktail Napkin is a really good example of a really bad explanation. There are so many things that are misleading, at best, and flat-out wrong, at worst, it's hard to know where to begin (or where to stop). Nevertheless, I'll try to keep it brief [I failed in that endeavor. — njg].

The author applies the equation:

\begin{equation} E = λ \end{equation}

Why? What is that equation? We don't know because the author does not say yet what all the symbols mean. It's all very well to impress people by slinging equations around, but it's more important to say what the equations actually mean. After all, the author might have chosen the wrong one.

Hotsos Symposium 2012

Time Bandits: How to Analyze Fractal Query Times

Tues, March 6, 2012 @ 2:15 pm

That's the title of my presentation at this year's Hotsos Symposium and no, I won't be trying to make any obscure connections between Terry Gilliam's famous movie and Oracle database products (as interesting as that exercise might be).

Instead, I'll be talking about fractals in time and how they can impact performance—especially Oracle database performance. The responsiveness of your Oracle application can be lost for longer than expected periods of time, ostensibly stolen by time bandits.

Preview Slides (2012). A more detailed explanation of the fractal technique used is now provided in the Guerrilla Data Analytics (GDAT) class: How to Get Beyond Monitoring from Linear Regression to Machine Learning.

Green Disk Sizing

I finally got around to completing item 5 on my 2011 list concerning electrical power consumed by a magnetic hard disk drive (HDD). The semi-empirical statement is:

Power ∝ N_platters × Ω^2.8 × D^4.6    . . .    (1)

where N_platters is the number of platters on the spindle, Ω is the rotational speed in revolutions per minute (RPM) and D the platter diameter in inches. The power consumed is then measured in Watts.

In principle, this makes (1) valuable for doing green HDD storage capacity planning. The bad news is, it is not in the form of an equation but a statement of proportionality, so it can't be used to calculate anything as it stands. More on that shortly. The good news is that all of the quantities in (1) can be read off from the data sheet of the respective disk vendor^†. Note that the disk capacity, e.g., GB (the usual capacity planning metric) does not appear in (1).

The outstanding question is: where do those funny non-integral exponents come from?

Throughput-Delay Curves

A colleague of mine at Yahoo.com asked me if I'd ever seen curves like this:

Not only is the answer, yes (it's a throughput-delay plot or XR plot in my notation), but that particular plot comes from my GCaP course notes. There, I use it to analyze the comparative performance of a functional multiprocessor (NS6000) and a symmetric multiprocessor (SC2000). Note how the two curves cross at around 1500 OPS. You can ask yourself why and if you can't come up with an explanation, you should be registering for a Guerrilla class. :)

The above XR plot also serves as a useful reminder that the throughput and response-time metrics are not only dependent on one another, but they are generally dependent in a nonlinear way—despite what some experts may claim:

My Year in Review 2011

Some days I wonder if I ever actually accomplish anything anymore. Maybe it's time to just pack it in and become a greeter at Walmart. I know a bit about how queues work, so that should put me a few notches ahead of the competition. And I would expect the competition to be fierce because it's a pretty cushy job; but not every day, apparently.

Before taking the big leap, I decided it might be a good idea to note down some of the technical projects I've worked on this year (over and above the daily grind):

A List of CaP Skills

This question popped up recently on Linkedin:

"Can someone tell me what skill set should a Performance and Capacity Analyst have and develop throughout his career?"

and I realized that, although I have a kind of list in my head, and I talk about such skills in my classes, I have been too lazy to write them down anywhere; which is pretty dumb. I must try to do something about that (New Year resolution? What are the odds?). In some ways, my fallback is the online Guerrilla Manual. Anyway, here is my (slightly edited) response to the LI question, and let it therefore constitute my first attempt at writing down such a list.

Season's Greetings 2011

Best wishes to all my blog readers and Guerrilla class alumni during this 2011 holiday season. I look forward to more of the same next year.

Image: The winter lights of Silicon Valley, California, from Mt. Hamilton webcam courtesy of UC Regents and Lick Observatory.