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Diffstat (limited to 'src/yqueue.hpp')
| -rw-r--r-- | src/yqueue.hpp | 195 |
1 files changed, 195 insertions, 0 deletions
diff --git a/src/yqueue.hpp b/src/yqueue.hpp new file mode 100644 index 0000000..56046da --- /dev/null +++ b/src/yqueue.hpp @@ -0,0 +1,195 @@ +/* + Copyright (c) 2007-2013 Contributors as noted in the AUTHORS file + + This file is part of 0MQ. + + 0MQ is free software; you can redistribute it and/or modify it under + the terms of the GNU Lesser General Public License as published by + the Free Software Foundation; either version 3 of the License, or + (at your option) any later version. + + 0MQ is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public License + along with this program. If not, see <http://www.gnu.org/licenses/>. +*/ + +#ifndef __ZMQ_YQUEUE_HPP_INCLUDED__ +#define __ZMQ_YQUEUE_HPP_INCLUDED__ + +#include <stdlib.h> +#include <stddef.h> + +#include "err.hpp" +#include "atomic_ptr.hpp" + +namespace zmq +{ + + // yqueue is an efficient queue implementation. The main goal is + // to minimise number of allocations/deallocations needed. Thus yqueue + // allocates/deallocates elements in batches of N. + // + // yqueue allows one thread to use push/back function and another one + // to use pop/front functions. However, user must ensure that there's no + // pop on the empty queue and that both threads don't access the same + // element in unsynchronised manner. + // + // T is the type of the object in the queue. + // N is granularity of the queue (how many pushes have to be done till + // actual memory allocation is required). + + template <typename T, int N> class yqueue_t + { + public: + + // Create the queue. + inline yqueue_t () + { + begin_chunk = (chunk_t*) malloc (sizeof (chunk_t)); + alloc_assert (begin_chunk); + begin_pos = 0; + back_chunk = NULL; + back_pos = 0; + end_chunk = begin_chunk; + end_pos = 0; + } + + // Destroy the queue. + inline ~yqueue_t () + { + while (true) { + if (begin_chunk == end_chunk) { + free (begin_chunk); + break; + } + chunk_t *o = begin_chunk; + begin_chunk = begin_chunk->next; + free (o); + } + + chunk_t *sc = spare_chunk.xchg (NULL); + free (sc); + } + + // Returns reference to the front element of the queue. + // If the queue is empty, behaviour is undefined. + inline T &front () + { + return begin_chunk->values [begin_pos]; + } + + // Returns reference to the back element of the queue. + // If the queue is empty, behaviour is undefined. + inline T &back () + { + return back_chunk->values [back_pos]; + } + + // Adds an element to the back end of the queue. + inline void push () + { + back_chunk = end_chunk; + back_pos = end_pos; + + if (++end_pos != N) + return; + + chunk_t *sc = spare_chunk.xchg (NULL); + if (sc) { + end_chunk->next = sc; + sc->prev = end_chunk; + } else { + end_chunk->next = (chunk_t*) malloc (sizeof (chunk_t)); + alloc_assert (end_chunk->next); + end_chunk->next->prev = end_chunk; + } + end_chunk = end_chunk->next; + end_pos = 0; + } + + // Removes element from the back end of the queue. In other words + // it rollbacks last push to the queue. Take care: Caller is + // responsible for destroying the object being unpushed. + // The caller must also guarantee that the queue isn't empty when + // unpush is called. It cannot be done automatically as the read + // side of the queue can be managed by different, completely + // unsynchronised thread. + inline void unpush () + { + // First, move 'back' one position backwards. + if (back_pos) + --back_pos; + else { + back_pos = N - 1; + back_chunk = back_chunk->prev; + } + + // Now, move 'end' position backwards. Note that obsolete end chunk + // is not used as a spare chunk. The analysis shows that doing so + // would require free and atomic operation per chunk deallocated + // instead of a simple free. + if (end_pos) + --end_pos; + else { + end_pos = N - 1; + end_chunk = end_chunk->prev; + free (end_chunk->next); + end_chunk->next = NULL; + } + } + + // Removes an element from the front end of the queue. + inline void pop () + { + if (++ begin_pos == N) { + chunk_t *o = begin_chunk; + begin_chunk = begin_chunk->next; + begin_chunk->prev = NULL; + begin_pos = 0; + + // 'o' has been more recently used than spare_chunk, + // so for cache reasons we'll get rid of the spare and + // use 'o' as the spare. + chunk_t *cs = spare_chunk.xchg (o); + free (cs); + } + } + + private: + + // Individual memory chunk to hold N elements. + struct chunk_t + { + T values [N]; + chunk_t *prev; + chunk_t *next; + }; + + // Back position may point to invalid memory if the queue is empty, + // while begin & end positions are always valid. Begin position is + // accessed exclusively be queue reader (front/pop), while back and + // end positions are accessed exclusively by queue writer (back/push). + chunk_t *begin_chunk; + int begin_pos; + chunk_t *back_chunk; + int back_pos; + chunk_t *end_chunk; + int end_pos; + + // People are likely to produce and consume at similar rates. In + // this scenario holding onto the most recently freed chunk saves + // us from having to call malloc/free. + atomic_ptr_t<chunk_t> spare_chunk; + + // Disable copying of yqueue. + yqueue_t (const yqueue_t&); + const yqueue_t &operator = (const yqueue_t&); + }; + +} + +#endif |