Improve packet timing method

[aturner]

Create another timing method to replace gettimeofday() and nanosleep() which aren't very granular fast on many platforms.

So I've determined the biggest problem: 1usec resolution is not enough. Some simple math shows that @ 125,000pps, the interval is 8usec. But at 130,000pps, we're looking at 7.7usec. Integer math converts that to 7usec and we end up doing around 141,000pps.

The answer seems to be that use floating point math and then for every 1/10 of a usec above the integer value, you sleep for an extra 1usec. Using the 130Kpps example of 7.7usec, I'd sleep for 7usec 3 times and 8usec 7 times, thus averaging 7.7usec. This seems to be pretty simple for calculating pps, but I still need to come up with an algorithm for more dynamic timing models (Mbps and multipliers). Rounding might be the easiest/best I can do.

[aturner]

A much simpler and accurate method is to read the RDTSC (ReaD Time Stamp Counter) counter for Pentiums:

• http://tldp.org/HOWTO/IO-Port-Programming-4.html
• http://www-unix.mcs.anl.gov/~kazutomo/rdtsc.html

Of course, this isn't the most cross-platform solution, but PPC/UltraSparc may have similar solutions and they're less popular then x86. Anyways, this looks like the way to go for now.

[aturner]

Initial tests using RDTSC did not provide any performance benefit. Will postpone until later. May need to use feature branch for this.

[aturner]

(In [1878]) create features area. refs #41

[aturner]

(In [1879]) create performance branch of trunk. refs #41

[aturner]

(In [1880]) commit work in progress. this doesn't seem to actually work though. :( Anyways, this just commits my work up to this point, but at some point it's going to need to be reworked. refs #41

[aturner]

According to Wikipedia, the RDTSC is inaccurate on multi-cpu/core systems and systems implementing SpeedStep?. Also, the incrementing rate may not equal the clock speed on P4, XEON and Core2Duo CPU's.

Interestingly, recent Linux 2.6 kernels may use a better timer HPET for gettimeofday() calls if the kernel is so configured. Ideally, I should use the HPET, but it looks more OS dependent.

[aturner]

another option is to try to use (p)select/poll/kqueue, etc for timing. Good chance that they on the back end use high resolution timers.

[aturner]

interesting trick: http://c-faq.com/osdep/sd25.html

[anonymous]

So I've determined the biggest problem: 1usec resolution is not enough. Some simple math shows that @ 125,000pps, the interval is 8usec. But at 130,000pps, we're looking at 7.7usec. Integer math converts that to 7usec and we end up doing around 141,000pps.

The answer seems to be that use floating point math and then for every 1/10 of a usec above the integer value, you sleep for an extra 1usec. Using the 130Kpps example of 7.7usec, I'd sleep for 7usec 3 times and 8usec 7 times, thus averaging 7.7usec. This seems to be pretty simple for calculating pps, but I still need to come up with an algorithm for more dynamic timing models (Mbps and multipliers). Rounding might be the easiest/best I can do.

[aturner]

(In [1970]) sync up my test code so far. refs #41

[aturner]

(In [1971]) switch to timespec (nanosecond precision) for sleeping. OS X's AbsoluteTime? methods are really damn accurate! refs #41

[aturner]

(In [1972]) writing to the ioport is complex enough it prolly doesn't belong full bore in the header. refs #41

[aturner]

(In [1973]) add support for rounding timespec's to usec accuracy for non-absolute timing methods refs #41

[aturner]

(In [1974]) now I calculate how much time has passed since the last packet was sent, but I still need to use the sleep-accellerator. refs #41

[aturner]

(In [1975]) cleanup refs #41

[aturner]

(In [1976]) minor cleanup. refs #41

[aturner]

(In [1977]) fix compile issues under linux refs #41

[aturner]

(In [1978]) minor tweaks refs #41

[aturner]

calculating the time passage [1974] is broken because it doesn't take into account the amount of time it takes to send the packet which is sufficently non-zero to cause serious problems.

[aturner]

(In [1979]) fix get_delta_time() to take in account of packet writing time. refs #41

[aturner]

(In [1980]) fix compile on non-OSX systems. refs #41

[aturner]

(In [2005]) clean up. refs #41

[aturner]

(In [2006]) merge features/performance -r1879:2005 to trunk. and cleanup. refs #41

[aturner]

(In [2008]) we dont use that anymore. refs #41

[aturner]

(In [2011]) disable to the old accrate test. refs #41

[aturner]

(In [2014]) default is now gtod() refs #41

[aturner]

done for now i think.