Is Parallel Programming Hard, And, If So, What Can You Do About It?

5.1. PARTITIONING EXERCISES 51R1R7 R6 R5 R4L0 R1 R2 R3Index LDEQ 0 DEQ 1 DEQ 2 DEQ 3Index RL−4 L−3 L−2L−8 L−7 L−6L−1L−5Enq 3RFigure 5.8: Hashed Double-Ended Queue With 12ElementsR4 R1 R2 R3DEQ 0 DEQ 1 DEQ 2 DEQ 31 struct pdeq {2 spinlock_t llock;3 int lidx;4 spinlock_t rlock;5 int ridx;6 struct deq bkt[DEQ_N_BKTS];7 };Index LEnq 3L1RIndex RFigure 5.9: Lock-Based Parallel Double-EndedQueue Data StructureIndex LR4L0R5R1R2L−2R3L−1DEQ 0 DEQ 1 DEQ 2 DEQ 3Index RFigure 5.7: Hashed Double-Ended Queue After Insertionsqueue holds a one-quarter slice of the full paralleldouble-ended queue.Figure 5.9 shows the corresponding C-languagedata structure, assuming an existing struct deqthat provides a trivially locked double-ended-queueimplementation. This data structure contains theleft-handlockonline2, theleft-handindexonline3,the right-hand lock on line 4, the right-hand indexon line 5, and, finally, the hashed array of simplelock-based double-ended queues on line 6. Ahigh-performance implementation would of courseuse padding or special alignment directives to avoidfalse sharing.Figure 5.10 shows the implementation of the enqueueand dequeue functions. 1 Discussion will focus1 One could easily create a polymorphic implementation inany number of languages, but doing so is left as an exerciseon the left-hand operations, as the right-hand operationsare trivially derived from them.Lines 1-13 show pdeq_dequeue_l(), which leftdequeuesand returns an element if possible, returningNULL otherwise. Line 6 acquires the left-handspinlock, and line 7 computes the index to be dequeuedfrom. Line 8 dequeues the element, and, ifline 9 finds the result to be non-NULL, line 10 recordsthe new left-hand index. Either way, line 11 releasesthe lock, and, finally, line 12 returns the element ifthere was one, or NULL otherwise.Lines 15-24 shows pdeq_enqueue_l(), which leftenqueuesthe specified element. Line 19 acquires theleft-hand lock, and line 20 picks up the left-handindex. Line 21 left-enqueues the specified elementonto the double-ended queue indexed by the lefthandindex. Line 22 updates the left-hand index,and finally line 23 releases the lock.As noted earlier, the right-hand operations arecompletely analogous to their left-handed counterparts.Quick Quiz 5.4: Is the hashed double-endedqueue a good solution? Why or why not?5.1.2.4 Compound Double-Ended QueueRevisitedThis section revisits the compound double-endedqueue, using a trivial rebalancing scheme that movesall the elements from the non-empty queue to thenow-empty queue.Quick Quiz 5.5: Move all the elements to thefor the reader.