Date: Wed, 7 Sep 2011 12:41:54 -0600 From: Manish Vachharajani <manishv@lineratesystems.com> To: Peter Jeremy <peterjeremy@acm.org> Cc: freebsd-hackers@freebsd.org Subject: Re: excessive use of gettimeofday(2) and other syscalls Message-ID: <CAHRgBhSxiN=KBvN=Ve5cmXY_PtCuivQY1jC87O-D7QRE9qiOqA@mail.gmail.com> In-Reply-To: <20110907061741.GC96277@server.vk2pj.dyndns.org> References: <20110906220115.GA25048@freebsd.org> <CAHRgBhRe8n%2BV3nSzRn4_fctHB1nie2ACk7oRVOPJqqKaMUgKrg@mail.gmail.com> <CAF6rxg=kzHP4zr_=LGnJDUQu-xEwgpy6QN=Lk4jqXa6hs=epKg@mail.gmail.com> <CAHRgBhT%2BKi%2BYPiK%2Bhn=fJ91eA=31tOaTPe_5xLSHQawa=%2BFD0Q@mail.gmail.com> <CAHwLALMwOu8wc5W03dar5fhkWkjN6f7eDGAT4a1u%2BK4eHHSsng@mail.gmail.com> <20110907061741.GC96277@server.vk2pj.dyndns.org>
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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 <peterjeremy@acm.org> wrote: > On 2011-Sep-06 16:44:48 -0600, Manish Vachharajani <manishv@lineratesyste= ms.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). =A0As 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(). =A0This is intended to be much cheaper > than gettimeofday(). > > On 2011-Sep-06 21:15:55 -0400, Rayson Ho <raysonlogin@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. =A0In order to reduce the cost where the resolution isn't > important, FreeBSD provides several non-standard clock types for > clock_gettime(2). =A0This 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 > --=20 Manish Vachharajani Founder LineRate Systems manishv@lineratesystems.com (609)635-9531 M
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