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>
next in thread | previous in thread | raw e-mail | index | archive | help
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>
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200403220657.i2M6vCrS097750>