Date: Fri, 12 Apr 2013 20:23:04 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: "Kenneth D. Merry" <ken@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r249334 - head/usr.bin/ctlstat Message-ID: <20130412200948.A1222@besplex.bde.org> In-Reply-To: <20130411224634.GA56177@nargothrond.kdm.org> References: <201304101601.r3AG1jZq083572@svn.freebsd.org> <20130411163712.Y1200@besplex.bde.org> <20130411224634.GA56177@nargothrond.kdm.org>
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On Thu, 11 Apr 2013, Kenneth D. Merry wrote: > On Thu, Apr 11, 2013 at 17:06:44 +1000, Bruce Evans wrote: >> ... >> It is now clear that this is still too verbose, since cur_secs and prev_secs >> are not used except to initialize etime. Simplifying this gives: >> >> etime = ctx->cur_time.tv_sec - ctx->prev_time.tv_sec + >> (ctx->prev_time.tv_nsec - ctx->cur_time.tv_nsec) * 1e-9; >> >> This might need casting to double of ctx->cur_time.tv_sec, in case time_t >> is unsigned and the time went backwards. Otherwise, this should be the >> usual expression for subtracting timespecs. > > The time can't go backwards in this case, because it is the system uptime. I expected this, but was not sure where the timestamps are generated. > Using wall clock time causes problems measuring performance when NTP > decides that the system time needs to change. I have a local patch to dd > to fix instances of bogus performance numbers due to its using wall clock > time to measure elapsed time. All uses of gettimeofday() are suspect. A quick grep shows other obviously wrong ones in ping*, time and systat. Uses of CLOCK_REALTIME are hopefully correct, but about half of them in /usr/src/*bin seem to be incorrect. ktrace and truss are interesting. Real times are used and are probably best for absolute times but wrong for relative times. Bruce
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