efun::eval_cost returns inaccurate numbers

[Voltara]

Evaluation cost is tracked internally by a CPU clock alarm; the eval_cost efun returns an estimate based on elapsed wall time. This can even return negative numbers to the caller. Proposed fix is to add an optional POSIX timers based eval_cost mechanism, and to use timer_gettime() when querying the amount remaining.

setitimer/getitimer is not as suitable because getitimer only has 1/250 second resolution on current versions of Linux, which limits precision when profiling LPC code.

[thefallentree]

use http://www.kernel.org/doc/man-pages/online/pages/man2/timer_create.2.html ?

[Voltara]

Yes, that. In my prototype, I have it use a CLOCK_PROCESS_CPUTIME_ID timer configured to send SIGVTALRM, similar to what setitimer / uvalarm currently does. The advantage here is timer_gettime() can be used to get the remaining time with high precision.

My first attempt was simply to use getitimer(), but then I noticed it was counting up in 4000 microsecond increments. That would be granular for profiling code speed from within LPC.

It's working well; I just need to integrate the code more cleanly into fluffos.

[thefallentree]

http://www.kernel.org/doc/man-pages/online/pages/man2/clock_getres.2.html allows you to get precision.

maybe useful to print that thing out too.