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DragonFly kernel List (threaded) for 2003-07
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Re: Initial messaging / message-port infrastructure is in

From: Jeremy Messenger <mezz7@xxxxxxx>
Date: Sun, 20 Jul 2003 12:23:00 -0500

On Sun, 20 Jul 2003 06:54:32 -0500, David Leimbach wrote:

> Ah... I need coffee. :)  Light Weight Kernel Threading perhaps? :)

Yes, all info are in the http://www.dragonflybsd.org/Goals/threads.cgi ..


> On Sunday, July 20, 2003, at 6:48AM, David Leimbach wrote:
>> Looks interesting... I need to get my FBSD box running again so I can
>> play with this
>> I suppose?  [I clobbered it a few days ago :)]
>> Anyway.. what does LWKT mean?  I somehow have missed it... I assume its
>> an
>> acronym?
>> Dave
>> On Saturday, July 19, 2003, at 8:56PM, Matthew Dillon wrote:
>>>     I've committed the initial messaging/message-port infrastructure.
>>>  Here
>>>     are the cvsweb links:
>>> http://www.dragonflybsd.org/cgi-bin/cvsweb.cgi/src/sys/kern/
>>> lwkt_msgport.c
>>> http://www.dragonflybsd.org/cgi-bin/cvsweb.cgi/src/sys/sys/msgport.h
>>> http://www.dragonflybsd.org/cgi-bin/cvsweb.cgi/src/sys/sys/thread.h
>>> http://www.dragonflybsd.org/cgi-bin/cvsweb.cgi/src/sys/kern/
>>> lwkt_msgport.c?rev=HEAD&content-type=text/x-cvsweb-markup
>>> http://www.dragonflybsd.org/cgi-bin/cvsweb.cgi/src/sys/sys/msgport.h?
>>> rev=HEAD&content-type=text/x-cvsweb-markup
>>> http://www.dragonflybsd.org/cgi-bin/cvsweb.cgi/src/sys/sys/thread.h?
>>> rev=HEAD&content-type=text/x-cvsweb-markup
>>>     None of it is tested yet since nothing uses it yet (though DEV is
>>> about to
>>>     start using it in a degenerate form), but I think people will be
>>>     interested in seeing all of those comments I was making about
>>> messaging
>>>     and IPIs realized by this commit.
>>>     This also shows off one of the big advantages of asynchronous IPI
>>>     messaging.  The messaging and port functions do not need to obtain
>>>     any mutexes (even if there were no MP lock) even when they wind up
>>>     queueing or dequeueing something.  But that isn't the only
>>> reason...
>>>     because the messaging operations are combined with scheduling ops
>>> and
>>>     one IPI will cover both the queueing and the scheduling aspects of
>>>     a message.
>>>     So right now we save at least 8 mutex equivalents with a single
>>> IPI
>>>     message:
>>>     Traditional mutex Design			LWKT/IPI design
>>>     cpu1: get_queue_mtx				cpu1: send IPIQ to target cpu cpu1:
>>>     queue_message
>>>     cpu1: rel_queue_mtx
>>>     cpu1: get_sched_mtx
>>>     cpu1: schedule_target_thread
>>>     cpu1: rel_sched_mtx
>>>     cpu1: (IPI the target cpu anyway
>>> 	  in case it is idle??)
>>>     cpu2: (receive wakeup IPI??)
>>>     cpu2: get_sched_mtx				cpu2: receive IPIQ cpu2:
>>>     locate_scheduled_thread		cpu2: queue message cpu2: rel_sched_mtx			
>>>     cpu2: schedule target thread cpu2: get_queue_mtx				cpu2: dequeue
>>>     message cpu2: dequeue_message			cpu2: execute message function
>>>     cpu2: rel_queue_mtx
>>>     cpu2: execute message function
>>>     Even if I were generous and ignored the fact that even in a
>>> mutexed
>>>     system cpu1 might want to wake up cpu2 with an IPI, the fact
>>> remains
>>>     that in a mutexed system *8* mutex operations are executed before
>>>     the message can be acted upon, and in the LWKT system *NO* mutex
>>>     operations are executed before the message can be acted upon.
>>>     The question then becomes: is the IPI latency (which costs
>>> neither cpu
>>>     any actual cycles but can take a 'long' time (e.g. 1uS)) and the
>>> IPI
>>>     interrupt overhead (on cpu2, which does eat cycles on cpu2) worth
>>> the
>>>     8 *CONTESTED* mutexes we just saved?  I would say:  probably.
>>>     And when you add in the other efficiencies.. for example, the
>>> fact that
>>>     lwkt_getport() and lwkt_waitport() (basically ALL message
>>> processing
>>>     involving the thread owning the message port in question)
>>> requires only
>>>     a critical section to manipulate.  No mutexes, no IPIs, no
>>> nothing.
>>>     Now is it worth it?  I think so!
>>> 						-Matt

bsdforums.org 's moderator, mezz.

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