Date: Wed, 7 Sep 2011 15:15:43 +0000 From: Alexander Best <arundel@freebsd.org> To: Peter Jeremy <peterjeremy@acm.org> Cc: freebsd-hackers@freebsd.org Subject: Re: excessive use of gettimeofday(2) and other syscalls Message-ID: <20110907151543.GA63310@freebsd.org> 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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On Wed Sep 7 11, Peter Jeremy wrote: > On 2011-Sep-06 16:44:48 -0600, Manish Vachharajani <manishv@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 <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. 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. chromium is triggering ~20.000 gettimeofday(2) calls per second on my machine. i'm running CURRENT on amd64. cheers. alex > > -- > Peter Jeremy
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