Comment by NekkoDroid
3 months ago
Doesn't really sound much better. You still load up the file(s) and the parser either way, so parsing all once vs on-demand is just a question of computation duration and considering how many config options are used the on-demand just seems really wasteful, especially after startup.
> load up the file
I/O is done piecewise, a line at a time. The file is never "loaded up". Again you're applying an intuition about how parsers are presented to college students (suck it all into RAM and build a big in-memory representation of the syntax tree) that doesn't match the way actual config file parsers work (read a line and interpret it, then read another).
I didn't mean it in a way that "all of the file is loaded into memory", just the parts you are always processing at the time (e.g. as you said line wise), which either way result in the same memory usage from the file being loaded.
The GP is correct in terms of super old systems.
In said systems, RAM was such an expensive resource that we had to save individual bits wherever we could. Such as only storing the last two digits of the year (aka the millennium bug).
The computational cost of infrequently rescanning the config files then freeing the memory afterwards was much cheaper than the cost of storing those config files into RAM. And I say “infrequently rescanning” because you weren’t talking about people logging in and out of TSSs at rapid intervals.
That all said, sshd was first written in the 90s so I find it hard to believe RAM considerations was the reason for the “first match” design of sshd’s config. More likely, it inherited that design choice from rsh or some other 1970s predecessor.
> hard to believe RAM considerations was the reason for the “first match” design of sshd’s config
And I repeat: first match involves less code. It's a simpler design. The RAM point was an interesting digression, I literally put it in parentheses!
I don’t think it does require less code. I don’t think it requires more code either. It’s just not a fundamental code change.
The difference is just either: overwriting values or exiting in the presence of a match. Either way it’s the same parser rules you have to write for the config file structure.
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