Date: Mon, 22 Mar 2004 15:57:12 +0900 From: Seigo Tanimura <tanimura@tanimura.dyndns.org> To: John Baldwin <john@baldwin.cx> Cc: Seigo Tanimura <tanimura@tanimura.dyndns.org> Subject: Re: Is MTX_CONTESTED evil? Message-ID: <200403220657.i2M6vCrS097750@shojaku.t.axe-inc.co.jp> In-Reply-To: <200403161009.48938.john@baldwin.cx> References: <200403160519.i2G5J0V6023193@urban> <200403161009.48938.john@baldwin.cx>
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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); } -- Seigo Tanimura <tanimura@tanimura.dyndns.org> <tanimura@FreeBSD.org>
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