Date: Mon, 1 Jul 2002 15:46:45 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Giorgos Keramidas <keramida@FreeBSD.ORG> Cc: audit@FreeBSD.ORG Subject: Re: sysconf(_SC_CLK_TCK) [NetBSD: basesrc/lib/libc/gen] Message-ID: <20020701142849.U6692-100000@gamplex.bde.org> In-Reply-To: <20020630040139.GC1223@hades.hell.gr>
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On Sun, 30 Jun 2002, Giorgos Keramidas wrote: > Does the following change look like a nice thing to make to our libc > too? I'm asking before trying to make a diff that does this, because > the log mentions that the standards do not require this to return the > value of HZ, and it's probably improper if programs depend on this > being == HZ. It's not only improper; it is just wrong. sysconf(_SC_CLK_TCK) has nothing to do with the clock that has frequency hz Hz. It is fuzzily related to the clock that has frequency stathz Hz. Under FreeBSD, sysconf(_SC_CLK_TCK) is always 128; stathz is always 128 on i386's except in early versions of FreeBSD-2 it was the same as hz and in FreeBSD-1 it didn't exist; stathz is different from 128 on some arches (e.g., on sparc64's it is the same as hz which may be user-configurable (default 100); hz is very arch-dependent and/or user-configurable (e.g., it is user-configurable on i386's (default 100), and decided by the hardware on alphas (usually 1024?)). > Date: Sun, 23 Jun 2002 22:19:03 +0300 (EEST) > From: "Perry E. Metzger" <perry@netbsd.org> > Subject: CVS commit: basesrc/lib/libc/gen > To: source-changes@netbsd.org > > Modified Files: > basesrc/lib/libc/gen: sysconf.c > > Log Message: > make sysconf(_SC_CLK_TCK) return the kernel hz value, instead of a > fixed constant, as is done on Solaris, Linux, etc. > > Technically, standards don't *require* this, but having it return a > constant is a violation of the spirit, and screws up programs that > (perhaps improperly) assume that it will return kernel hz. NetBSD should probably use stathz in preference to hz. The correct value for sysconf(_SC_CLK_TCK) is currently 1000000 or some multiple of that. Any other value causes gratuitous loss of precision due to double rounding in the one interface where sysconf(_SC_CLK_TCK) is used, namely times(3) (NetBSD also changed libc/gen/times.c to go with the above commit). times(3) is implemented using getrusage(2). The kernel starts with the total process time which has a precision of 2^-64 seconds in -current (1 usec in RELENG_4). It converts this to usec, so the precision is reduced to 1 usec in both -current and RELENG_4. The time is accurate to about 1 usec on current hardware, so 1 usec is adequate precision. The kernel then subdivides the total process time into user, system and interrupt portions according to the number of user, system and interrupt statclock ticks that occurred while the process was running. These ticks have a resolution of 1/stathz seconds, so the subdivision is fuzzy. The subdivision is a little less fuzzy in -current, since interrupt handling is pessimized by switching to a different process, so there are no interrupt ticks except for interrupt tasks. The kernel then reports the subdivided time to userland using getrusage(). times(3) just scales the subdivided time into units of 1/sysconf(_SC_CLK_TCK) seconds (it actually uses CLK_TCK instead of sysconf(_SC_CLK_TCK) in FreeBSD; this is fixed in NetBSD). It loses accuracy by up to 1000000/syscons(_SC_CLK_TCK) (typically 7812) usec. This is not very important, since statistical errors in the subdivision may lose even more accuracy. However, it would be nice if the sum of the user and system times were as accurate as possible. Changing sysconf(_SC_CLK_TCK) to stathz now would mainly prepare for changing it to a more sensible value later (clock_t needs to have more than 32 bits before sysconf(_SC_CLK_TCK) can have a sensibly large value), and this change can wait until 32 bit arches are not supported. No one uses times(3) anyway -- its man page says that it was obsoleted by getrusage(2) long ago. Note that CLK_TCK needs to be changed to sysconf(_SC_CLK_TCK) to go with this change, or better removed (it was knwn to be a mistake in the first version of POSIX and was removed in POSIX.1-2001). Note that changing CLK_TCK breaks binary compatibility, and removing breaks source compatibility with crufty sources starting with libc/gen/times.c. There are similar issues for ISO C's CLOCKS_PER_SEC. fanf pointed to a discussion in comp.std.c about C99's breakage of CLOCKS_PER_SEC. C99 requires it to be a constant expression, so it can't be #defined as sysconf(_SC_CLK_TCK). Nothing seems to have come of the discussion. Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-audit" in the body of the message
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