Date: Mon, 6 Dec 2010 13:01:13 -0500 From: John Baldwin <jhb@freebsd.org> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: freebsd-current@freebsd.org Subject: Re: Process accounting/timing has broken recently Message-ID: <201012061301.13647.jhb@freebsd.org> In-Reply-To: <20101206163830.GA53157@troutmask.apl.washington.edu> References: <20101205231829.GA68156@troutmask.apl.washington.edu> <201012060944.03196.jhb@freebsd.org> <20101206163830.GA53157@troutmask.apl.washington.edu>
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On Monday, December 06, 2010 11:38:30 am Steve Kargl wrote: > On Mon, Dec 06, 2010 at 09:44:03AM -0500, John Baldwin wrote: > > On Sunday, December 05, 2010 6:18:29 pm Steve Kargl wrote: > > > Sometime in the last 7-10 days, some one made a > > > change that has broken process accounting/timing. > > > > > > laptop:kargl[42] foreach i ( 0 1 2 3 4 5 6 7 8 9 ) > > > foreach? time ./testf > > > foreach? end > > > Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04 > > > 69.55 real 38.39 user 30.94 sys > > > Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04 > > > 68.82 real 40.95 user 27.60 sys > > > > > > testf is a numerically intensive program that tests the > > > accuracy of expf() in a tight loop. User time varies > > > by ~3 seconds on my lightly loaded 2 GHz core2 duo processor. > > > I'm fairly certain that the code does not suddenly grow/loose > > > 6 GFLOP of operations. > > > > The user/sys thing is a hack (and has been). We sample the PC at stathz (~128 > > hz) to figure out a user vs sys split and use that to divide up the total > > runtime (which actually is fairly accurate). All you need is for the clock > > ticks to fire just a bit differently between runs to get a swing in user vs > > system time. > > > > What I would like is to keep separate raw bintime's for user vs system time in > > the raw data instead, but that would involve checking the CPU ticker more > > often (e.g. twice for each syscall, interrupt, and trap in addition to the > > current once per context switch). So far folks seem to be more worried about > > the extra overhead rather than the loss of accuracy. > > > > John, > > Thanks for the comment. It seems this splitting has become > worse (for some definition of worse) in that previously the > user time variation was on the order of tenth of a second not > seconds. In thinking about the issue, I recalled that some > changes to npx.c were committed 10 days ago. Perhaps, there > is slightly more context switch overhead in dealing with the > FPU registers, and this has increased the sys time. Hmm, I wonder if the eventtimer stuff that has gone into HEAD recently could be a factor? It might change when statclock() is called. -- John Baldwin
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