This is great info, thanks. Is it worth having some kind of environment variable tunable (or even compile time tunable) to have a "fast" gettimeofday then? Is there a complimentary body of code that assumes gettimeofday is precise?
Manish On Wed, Sep 7, 2011 at 12:17 AM, Peter Jeremy <peterjer...@acm.org> wrote: > On 2011-Sep-06 16:44:48 -0600, Manish Vachharajani > <mani...@lineratesystems.com> wrote: >>Under 7.3 (haven't checked 8 or 9) this issue crops up because the >>time system call calls gettimeofday under the hood (see >>lib/libc/gen/time.c). As a result, the kernel tries to get an >>accurate subsecond resolution time that simply gets chopped to the >>nearest second. > > Under 8.x and later, time(3) uses clock_gettime(CLOCK_SECOND,...) > rather than gettimeofday(). This is intended to be much cheaper > than gettimeofday(). > > On 2011-Sep-06 21:15:55 -0400, Rayson Ho <raysonlo...@gmail.com> wrote: >>IMO, the time returned by gettimeofday does not need to be high >>precision. There are higher resolution time APIs on Linux and I >>believe the application programmers know when to use the slower but >>more accurate clock API. > > There are 3 standard APIs for returning time of day: > time(3) provides second precision > gettimeofday(2) provides microsecond precision > clock_gettime(2) provides nanosecond precision > > By default, FreeBSD attempts to provide resolution as close as > possible to the precision - which makes the 2 system calls fairly > expensive. In order to reduce the cost where the resolution isn't > important, FreeBSD provides several non-standard clock types for > clock_gettime(2). This approach differs from Linux - and it seems > that there is a non-trivial body of code that assumes that calling > gettimeofday() is very cheap. > > There is probably a good case for an API that provides a resolution > of the order of a tick but there is no standard for this. > > -- > Peter Jeremy > -- Manish Vachharajani Founder LineRate Systems mani...@lineratesystems.com (609)635-9531 M _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
