Date: Mon, 22 Mar 2004 14:20:39 -0500 From: John Baldwin <jhb@FreeBSD.org> To: Seigo Tanimura <tanimura@tanimura.dyndns.org> Cc: arch@FreeBSD.org Subject: Re: Is MTX_CONTESTED evil? Message-ID: <200403221420.39732.jhb@FreeBSD.org> In-Reply-To: <200403220657.i2M6vCrS097750@shojaku.t.axe-inc.co.jp> References: <200403160519.i2G5J0V6023193@urban> <200403161009.48938.john@baldwin.cx> <200403220657.i2M6vCrS097750@shojaku.t.axe-inc.co.jp>
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On Monday 22 March 2004 01:57 am, Seigo Tanimura wrote: > On Tue, 16 Mar 2004 10:09:48 -0500, > John Baldwin <john@baldwin.cx> said: > > john> On Tuesday 16 March 2004 12:19 am, Seigo Tanimura wrote: > >> _mtx_unlock_sleep() currently wakes up only one thread being blocked, > >> and leaves MTX_CONTESTED on a mutex. According to Solaris Internals, > >> that strategy adds an overhead to check for MTX_CONTESTED on a mutex, > >> even though it is not held by any thread. The thread waken up cannot > >> grab the mutex immediately by _obtain_lock() and have to go through > >> _mtx_lock_sleep(). The penalty tends to be large for a mutex with a > >> high contention, and we have at least one of such a mutex - Giant. > >> > >> What would it be like if we axed MTX_CONTEST and let > >> _mtx_unlock_sleep() wake up all of the blocked threads? > > john> We wouldn't be able to axe MTX_CONTEST. We also use it to determine > on unlock john> if we can unlock easily or if we have waiters that we need > to awake. The john> only way we might be able to axe MTX_CONTEST would be > to penalize every john> unlock operation requiring a turnstile lookup (spin > lock acquire/release + john> hash table lookup) even unlocks of an > uncontested mutex. However, what I john> think you want to do is get rid > of the mtx_lock == MTX_CONTESTED case and use john> turnstile_wakeup() > rather than turnstile_signal()? Is that what you are > > Yes. What I an wondering is whether the reduction of the cost due to > a mutex with waiters and no holders can beat the cost of waking up all > the waiters on the turnstile. > > > john> asking? That is something we can try at some point in the future, > but we john> would need to benchmark both ways. What we might can do is > add a kernel john> option MUTEX_WAKE_ALL or some such that uses the Solaris > behavior. Having it john> be an option like ADAPTIVE_MUTEXES makes it > easier to benchmark both cases. > > > On the detection of the waiters by MTX_CONTEST, maybe we can test > MTX_CONTEST on mtx_lock before performing _release_lock(). If the > test succeeds, _mtx_unlock_sleep() must be called and we do not need > to perform an atomic test-and-set. A race can occur if the mutex is > locked after the MTX_CONTEST test, but _release_lock() should then > cover the case. > > Pseudocode: > > mtx_unlock(m) > { > if (m->mtx_lock & MTX_CONTEST || !_release_lock(m)) > _mtx_unlock_sleep(m); > } I would just always do the atomic op to avoid penalizing the common fast case (not contested, not recursed). BTW, last week I did implement the MUTEX_WAKE_ALL kernel option in the jhb_lock p4 branch. I've been very busy with work stuff though for several days now, but when I get some free time again I'll post the patch so people can play with it. -- 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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