On Fri, 28 Oct 2005, David Xu wrote: >>> On the other hand, a lower risk change might be to simply add a new >>> CLOCK_ type for lower resolution, and have a timer synchronize a >>> variable to the system clock once every 1/10 of a second. This avoids >>> having to muck with VM layout, etc. >> >> Is the CLOCK_* namespace ours to muck about with in the first place ? >> > I prefer this way, can you implement it? The global page idea is a > complex, someone can slowly work on it, there are many things can be > done, for example, fast syscall using sysenter/sysexit. Just as an experiment, I added two new time counters: CLOCK_POOR - use a cached time returned by getnanotime(), which may be out of date by up to 10/hz (see phk's e-mail for the correct number). CLOCK_SECOND - Same as CLOCK_POOR, only truncate the nanoseconds field to 0, and return 1 second as the resolution via clock_getres(). Patch is attached. Performance measurements on a P4 Xeon, dual processor with UP and SMP kernel configurations, no debugging. Measured using 10,000 loops of each system call, 12 samples per clock type. Measurements are in nanoseconds, measured using CLOCK_REALTIME, which has a resolution of 280ns reported via clock_getres(). Not surprisingly, the two above clocks measure about the same (they should do), and are a lot faster than real time measurements (they should be). As currently implemented, gettimeofday() appears to cost the same as CLOCK_REALTIME. It would be interesting to distribution of clock wrongness was, but I'm not set up to measure that currently. Robert N M Watson x 7UP/gettimeofday + 7UP/clock_gettime_poor * 7UP/clock_gettime_realtime % 7UP/clock_gettime_second +--------------------------------------------------------------------------+ | _at_ * | | _at_ * | | _at_ * | | _at_ * | |+ _at_ * | |_at_ _at_ x* | |_at_%_at_ _at__at_ x* *** *| | |__A_M| |_AA|_| | +--------------------------------------------------------------------------+ N Min Max Median Avg Stddev x 12 1410 1507 1502 1493.75 26.574509 + 12 523 634 624 595.58333 47.586206 Difference at 95.0% confidence -898.167 +/- 32.632 -60.1283% +/- 2.18457% (Student's t, pooled s = 38.5399) * 12 1437 1627 1503 1507.6667 42.401401 No difference proven at 95.0% confidence % 12 524 631 623 595 45.696628 Difference at 95.0% confidence -898.75 +/- 31.6491 -60.1674% +/- 2.11877% (Student's t, pooled s = 37.379) x 7SMP/gettimeofday + 7SMP/clock_gettime_poor * 7SMP/clock_gettime_realtime % 7SMP/clock_gettime_second +--------------------------------------------------------------------------+ |_at_ _at_ * x*| |_at_ _at_ * x*| |_at_% _at_% * x*| |_at__at_ _at__at_ **x**| ||MA_M |AMM| +--------------------------------------------------------------------------+ N Min Max Median Avg Stddev x 12 1406 1443 1436 1425.4167 16.527984 + 12 503 566 517 527.08333 26.352362 Difference at 95.0% confidence -898.333 +/- 18.6239 -63.0225% +/- 1.30656% (Student's t, pooled s = 21.9957) * 12 1404 1450 1444 1430.0833 19.327128 No difference proven at 95.0% confidence % 12 505 562 549 531.5 26.182923 Difference at 95.0% confidence -893.917 +/- 18.538 -62.7127% +/- 1.30054% (Student's t, pooled s = 21.8943)
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