Saturday, December 3, 2016

Optimization and Performance

Some good thoughts here:

I am personally used to writing code where 100 CPU cycles matters. So invoking a function that acquires a lock by way of a shared-memory interlocked instruction that may take 100 cycles is something I am apt to think hard about; even more worrisome is if that acquisition could block waiting for 100,000 cycles. Indeed this situation could become disastrous under load. As you can tell, I write a lot of systems code. If you're working on a network-intensive application, on the other hand, most of the code you write is going to be impervious to 100 cycle blips, and more sensitive to efficient network utilization, scalability, and end-to-end performance. And if you're writing a little one-time script, or some testing or debugging program, you may get away with ignoring performance altogether, even multi-million cycle network round-trips.

To be successful at this, you'll need to know what things cost. If you don't know what things cost, you're just flailing in the dark, hoping to get lucky. This includes rule of thumb order of magnitudes for primitive operations – e.g. reading / writing a register (nanoseconds, single-digit cycles), a cache hit (nanoseconds, tens of cycles), a cache miss to main memory (nanoseconds, hundreds of cycles), a disk access including page faults (micro- or milliseconds, millions of cycles), and a network roundtrip (milliseconds or seconds, many millions of cycles) – in addition to peering beneath opaque abstractions provided by other programmers, to understand their best, average, and worst case performance.

Clearly the concerns and situations you must work to avoid change quite substantially depending on the class of code you are writing, and whether the main function of your program is delivering a user experience (where usability reigns supreme), delivering server-side throughput, etc. Thinking this through is crucial, because it helps avoid true "premature optimization" traps where a programmer ends up writing complicated and convoluted code to save 10 cycles, when he or she really needs to be thinking about architecting the interaction with the network more thoughtfully to asynchronously overlap round-trips. Understanding how performance impacts the main function of your program drives all else.

If I esteem mankind to be in error, shall I bear them down? No. I will lift them up, and in their own way too, if I cannot persuade them my way is better; and I will not seek to compel any man to believe as I do, only by the force of reasoning, for truth will cut its own way.

"Thou shalt love thy wife with all thy heart, and shalt cleave unto her and none else."

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