gettimeofday() -- user-space vs. system call

gettimeofday() -- user-space vs. system call

Posted Feb 7, 2007 9:26 UTC (Wed) by ekj (subscriber, #1524)
In reply to: gettimeofday() -- user-space vs. system call by AnswerGuy
Parent article: Comparing Linux and Minix

The balance tends to shift over time too.

The amount of memory available increases a lot more than the available syscalls do. In other words, putting the data that gettimeofday() needs into the address-space of every process costs just as many bytes now as it always did (well, modulus avg number of processess on a box), but the available memory increases exponentially.

Lots of stuff may make sense on a 16GB machine which doesn't make sense on a 4MB machine. If you do enough such trickery that you'd have wasted half the RAM of the 4MB machine you'll have wasted 0.01% of the memory of the large machine. If you assume the large machine has 10 times as many processes, you've still only wasted 0.1% of the available memory.

This is even more true for space/performance tradeoffs that are internal to the kernel only. Back when Linux was new, wasting 1MB to double in-kernel performance would've been ridicolous. Today it's equally obvious that it'd be a huge win in most scenarios.

Gets complicated though, because larger structures tend to hurt cache-hit ratio, and main-memory is *SLOW* compared to on-die-cache.

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