On Sun, 27 Nov 2005, Robert Watson wrote:
... Attached is a small LD_PRELOAD library to allow applications to be switched to the _FAST variants for gettimeofday().
% /* % * Small wrapper library to substitute implementations of gettimeofday(2) and % * time(3) with lower resolution variations. time(3) is unconditionally % * degraded, since it will return a truncated time anyway. gettimeofday(3) % * checks the TIMEWRAPPER environmental variable, which can be set to either % * "PRECISE" or "FAST". % */ time(3) should use the environment variable too, since the fast version gives a value that is both imprecise and wrong. It inherits bugs from the kernel's time_second variable. time_second is not the current time truncated, but is the (current time less up to about tc_tick/HZ) truncated. It lags the current time by more than 1 second for up to about tc_tick/HZ seconds before every rollover of the correct truncated time. When HZ was 100 it was very easy for applications to see time_second being wring since the average size of the window of incoherency was 1/200 second. time_second is visible in file times by default for ffs file times. The bug can be fixed for ffs too globally using the vfs.timestamp_precision sysctl. I only use this sysctl for running POSIX tests for this bug, but would be surprised if always using it caused noticeable pessimization. Updates of file timestamps are cached in non-broken file systems, so it is hard for them to occur as often as ones for context switches -- they will usually be limited by physical disk write bandwidth. Bruce _______________________________________________ cvs-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/cvs-all To unsubscribe, send any mail to "[EMAIL PROTECTED]"
