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There is also an opportunity for a minor optimization here If we have more than 61 events to wait on, we could remove m_b l o c ked F i b e rQu e u e NewEvent from our list and possibly wait on a sixty-second The m_b loc ked F i be rQu e u eNew E v e n t event, as we'll see, is set only when we'd like another blocked thread to wake up and try to accumulate more HANDL Es for its wait Since we already have a full set, there is no need to for this thread to participate Finally, there is one other design decision that is worth contemplating Notice that we only check to see whether a wait has been satisfied when the work queue becomes empty It might be worth checking HANDLEs occasion ally, perhaps with a a timeout instead of I N F I N I T E , so that we don't starve blocked callbacks in favor of always running newly enqueued work This solution wouldn't complicate the implementation too much We'd just peri odically run the existing blocking logic with a different timeout We've almost enumerated all of the details Nobody said building a cus tom VMS would be easy We need to look at what happens when the wait returns There are four basic success cases 1 If the wait returned because the shutdown event was set (dwRet equals WAI T_O B J E CT_a), we can immediately return NU L L We don' t bother worrying about the fact that the blocked fiber queue is now missing entries (since we dequeued them) because the pool is termi nating anyway Both the F i b e rWo r k Rout i n e and B l o c k method check the shutdown flag, so they will do the right thing when we return 2 If the wait returned due to new work arriving in the work queue (dwRet equals WAIT_O B J E CT_a + 1), we will enqueue the blocking information we removed back into the queue so other threads can wait on these events instead, set the m_b l o c ked F i b e rQueueNewEvent so threads that are already waiting can add the HAND L E s to their wait set, and then go back around our loop to retrieve the work from the queue and run it 3 If we were awakened because the blocked fiber event was set (dwRet equals WAIT_O B J E CT_a + 2), this is just a hint by another thread that we should rebuild our wait list While there are opportunities for optimization here, we currently loop back around and execute the
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same logic above If we find the work queue is empty, we'll rebuild our wait set and reissue the wait 4 Finally, we may have been awakened because one of the blocked fibers' HANDLEs was signaled If that is the case, we will just add all of the removed waits back to the blocked fiber queue, minus the one that woke up, and switch to the awakened fiber so it can execute When we do this, we pass the calling fiber's F i berState as m_pWa k i n g F i b e r As we saw earlier in the Block routine, this causes the awakened fiber to enqueue the calling fiber back into the runnable list We do this so that if subsequent work is found and a runnable fiber is needed, the afore mentioned logic will find this particular fiber and pass the work to it And finally, we omit any detailed discussion of how to handle errors (Also note that we make no special mention of WAIT_ABANDON E D_e Using mutexes in a fiber based system is a little silly because they imply thread affinity) Here's the code that implements all of this logic, concluding the ContextSwi t c h function
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if ( WAIT_OB J ECT_a <= dwRet && dwRet < ( WAIT_OB J E CT_a + cDequeued F i bers + c Re s e rved { i n t index = dwRet - WAIT_OBJ ECT_a j if ( i ndex == a ) I I W e got t h e s h u tdown s i g na l Terminate immed iately ret u r n NU L L j } e l s e i f ( i ndex = = 1 I I index = = 2 ) { II E i t he r new wor k a r rived at t h e q u e u e or a d d i t i o n a l I I wa i t s we re added R e s t o r e t h e q u e u e a n d then loop I I b a c k a round to d i s pat c h the work or rega t h e r wait s if ( c Deq ueued F i bers > a ) { E n t e r C r it i c a lSect ion ( &m_b l o c k ed F i b e rQueueC r st ) j for ( i nt i = a j i < cDeq u e u e d F i bers j i++ ) m_p B l o c k e d F i be rQueue - > p u s h_front ( p pDeq ueued F i bers [ i ] ) j LeaveCrit i c a lSection ( &m_blocked F i b e rQueueC r st ) j I I Not ify ot her t h re a d s t h e r e a re ava i l a b l e wait s if ( i ndex == 1 ) SetEvent ( m_bloc ked F i berQueueNewEvent ) j }
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