There's an old joke that, if you think about it, you can apply directly to system bottlenecks.
Two hikers are walking through the woods when they come face-to-face with a pack of wolves. One of the hikers immediately drops to the ground and hastily changes from his hiking boots to the running shoes he had in his backpack.
The 2nd hiker says, "What are you doing ! You can't outrun those wolves!"
The 1st replies, "I don't have to outrun those wolves. I just have to outrun you."
Web developers tend to know their system's biggest bottleneck, but how often do you know the 2nd biggest one? Right, in one sense it doesn't matter - because the 2nd bottleneck doesn't get to become a bottleneck unless it's the biggest one.
There is an economic model hidden within every complex system. This includes something as mundane as web system performance. Knuth famously said (or re-said) that "premature optimization is the root of all evil" which could be restated as - if you optimize before you know what needs it, you're optimizing (and probably breaking) the wrong thing.
Hence we don't (or aren't supposed to) optimize what doesn't need it. Seems obvious - but it has interesting ramifications.
When something doesn't need optimizing, we can afford to be (and often tend to be) lazy with it when it comes to performance. Concretely, if your code's database access is going to take a 15 milliseconds, worrying that processing that data will take 20 microseconds because of your sloppy n^2 algorithm probably isn't worth much thought.
If that statement raises your ire, feel free to sit in your chair and pout - because there are thousands of websites that were happily coded using notepad with interpreted and dynamic scripting languages that flagrantly use gotos and lists as if they were hashmaps. I've seen it. It's enough to turn your stomach. It's not pretty.
For the average, everyday web hardware ecosystem - we have CPU power to spare. And in the bigger business sense, if I can save time developing a website cutting performance-concerned corners with no ramifications, all the better.
Web development largely started in scripting languages (i.e. perl-cgi, php). Again for the same reason - CPU to spare as compared to other bottlenecks.
In fact, I'll go so far as to say that the popularity of scripting language web frameworks required the condition that disks be some order of magnitude slower than CPUs. That's right - I'm looking at you Rails, Grails, Nails, and Jails.. (ok, not Jails, it's a Java web framework but it rhymed).
Java web frameworks added a lot of structure, verbosity, and performance that simply wasn't needed (and eventually, amazing bloat). If your bottleneck was the database/disk - your web processing simply had to not add significantly to that - and regardless of the language, that wasn't hard.
A simple definition of latency is the time it takes to get data back after requesting it. Similarly to an program anyway, bandwidth could be viewed as how long it takes us to get all the data requested (once you start getting any).
Think of how that relates to code performance. If your latency is 3ms (a reasonable server harddisk seek time) - it doesn't matter if your code is hand-loved machine language or interpreted COBOL - it does nothing for that 3ms. In CPU time, 3ms is an eternity.
As a general tendency however, the more data you receive, the more processing that likely goes around it. Consider a few megabyte JSON message - at a minimum it will likely be parsed. Possibly shoved into a map or an object.
Said another way - lowering latency and increasing bandwidth will tend to put more pressure on processing data (i.e. requiring more CPU/code performance)
So all this time we're happily and harshly slowed down by slow things like spindly harddisk drives and networks. Then, in walk Solid State Drives. Prices and capacities are both heading in the normal directions for new technology (down and up, respectively).
Latency goes from standard spindle drive 3ms seeks to (varying reports) 100microsecond seeks.
Argue the specifics if you will, but for some number of existing systems, installing an SSD will remove the database as the primary bottleneck. In fact, this is probably the cheapest way to improve your system's performance today.
What happens to the bottleneck in those systems? It will shift somewhere else (i.e. the SSD put on its running shoes). In many cases, it will shift to the CPU (CPU in this case is a polite way of saying "your code").
Everyday across the world, there are meetings at companies complaining about the performance of their website. Today, many of those say "get the DBA in here".
In some of those meetings soon, the shift will be away from blaming the database. Some will push for code optimization (postmature), some for bigger hardware, and some for faster languages.
Keep in mind, this is a subtle, slow moving effect. Having your CPUs pegged all the time might not make you change anything today but may make you reconsider your architecture next time you build something.
Of course the network is a bottleneck too - at least for now. In places like Korea and Kansas City that's not so true. If you haven't heard, if you live in Kansas City you can get Google Fiber to your home. In other words, your internet speed will be 100 times faster than the average internet in the US. (In fact, if your machine has the common SATA2 disk drive interface, sending a file to your neighbor across town in Kansas City will only take about 3 times as long as storing it on your own disk just a few inches away).
Here's another prediction - in 5 years the phrase "downloading a movie" won't exist. (We used to say we were "downloading an image" which was preceded by us saying we were just "downloading").
If bandwidth drastically increases, it will change how we write code. We think nothing of loading a 1M webpage now which 10 years ago was offensive. In the future, we may think the same thing about a 100M webpage.
Given that data expands to fill available bandwidth (modified Parkinson's Law) our programs will tend to process much more data. Processing speed will matter more and more.
And the more often code becomes the bottleneck, the more often solutions to fix that will be considered.
Simply - your favorite bottlenecks might be changing. And for that to happen, your disk doesn't necessarily need to be able outrun your CPU - it just has to be able to outrun your code. (And it wouldn't hurt if it could also outrun, you know, wolves too).
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