Date: Tue, 06 Jan 2004 10:21:23 -0500 (EST) From: John Baldwin <jhb@FreeBSD.org> To: Bruce Evans <bde@zeta.org.au> Cc: current@freebsd.org Subject: Re: Expensive timeout(9) function ? Message-ID: <XFMail.20040106102123.jhb@FreeBSD.org> In-Reply-To: <20040105175530.K4057@gamplex.bde.org>
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On 05-Jan-2004 Bruce Evans wrote: > On Sun, 4 Jan 2004, Bjoern A. Zeeb wrote: > >> what reports do you expect with the >> >> "Expensive timeout(9) function" >> >> message ? > > What you reported (function names and timeout time) is interesting. > > Why do we see it ? > > Kernel bugs :-). > >> Expensive timeout(9) function: 0xc04885a0(0) 1.024846430 s [1] >> Expensive timeout(9) function: 0xc04885a0(0) 1.024846430 s [1] >> Expensive timeout(9) function: 0xc04b3940(0) 0.008629758 s [2] >> Expensive timeout(9) function: 0xc04b39a0(0) 0.004333781 s [2] >> Expensive timeout(9) function: 0xc04f71f0(0) 0.027004551 s [3] >> Expensive timeout(9) function: 0xc04f71f0(0) 0.027004551 s [3] >> Expensive timeout(9) function: 0xc04f71f0(0) 0.027004551 s [3] >> >> [1] sys/kern/kern_synch.c:loadav() >> [2] sys/kern/uipc_domain.c:pfslowtimo() >> [3] sys/netinet/ip_fw2.c:ipfw_tick() > > [1] is easiest to understand. loadav() is obviously broken since it uses > sleep locks. Apparently it sometimes sleeps for more than 1 second > altogether! There is a check for sleeping in timeouts under DIAGNOSTIC. > I would expect to complaints from this too if you just used DIAGNOSTIC > to get the above. > > [3] ipfw_tick() is obviously broken in the same way. This is from > blind conversion of splimp() to a sleep lock. Mutexes work quite > differently from spl's. A quick fix for timeout routines that only > lock things once might be to use mtx_trylock() and not do anything in > the timeout routine (except re-arm the timeout, perhaps with a smaller > interval) if the mutex cannot be acquired immediately. This depends > on the exact timing of timeout routines not being critical (not that > we have exact timing -- the above shows all timeouts being delayed by > a factor of at least 100 (1 second instead of 1/100 seconds)). This > should work expecially well in loadav() -- loadav() intentionally adds > jitter to the interval. This might have worked in schedcpu() too > (schedcpu() was converted to a thread). Ugh, loadav() needs to move to a thread, too, then. Perhaps loadav() and schedcpu() can share a thread by having the schedcpu thread just run loadav() occasionally. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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