1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
|
/*
Copyright (c) 2007-2009 FastMQ Inc.
This file is part of 0MQ.
0MQ is free software; you can redistribute it and/or modify it under
the terms of the Lesser GNU 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
Lesser GNU General Public License for more details.
You should have received a copy of the Lesser GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#ifndef __ZMQ_YPIPE_HPP_INCLUDED__
#define __ZMQ_YPIPE_HPP_INCLUDED__
#include "atomic_ptr.hpp"
#include "yqueue.hpp"
#include "platform.hpp"
namespace zmq
{
// Lock-free queue implementation.
// Only a single thread can read from the pipe at any specific moment.
// Only a single thread can write to the pipe at any specific moment.
//
// T is the type of the object in the queue.
// If the template parameter D is set to true, it is quaranteed that
// the pipe will die in a finite time (so that you can swich to some
// other task). If D is set to false, reading from the pipe may result
// in an infinite cycle (if the pipe is continuosly fed by new elements).
// N is granularity of the pipe (how many elements have to be inserted
// till actual memory allocation is required).
template <typename T, bool D, int N> class ypipe_t
{
public:
// Initialises the pipe. In D scenario it is created in dead state.
// Otherwise it's alive.
inline ypipe_t () :
stop (false)
{
// Insert terminator element into the queue.
queue.push ();
// Let all the pointers to point to the terminator
// (unless pipe is dead, in which case c is set to NULL).
r = w = &queue.back ();
c.set (D ? NULL : &queue.back ());
}
// Following function (write) deliberately copies uninitialised data
// when used with zmq_msg. Initialising the VSM body for
// non-VSM messages won't be good for performance.
#ifdef ZMQ_HAVE_OPENVMS
#pragma message save
#pragma message disable(UNINIT)
#endif
// Write an item to the pipe. Don't flush it yet.
inline void write (const T &value_)
{
// Place the value to the queue, add new terminator element.
queue.back () = value_;
queue.push ();
}
#ifdef ZMQ_HAVE_OPENVMS
#pragma message restore
#endif
// Flush the messages into the pipe. Returns false if the reader
// thread is sleeping. In that case, caller is obliged to wake the
// reader up before using the pipe again.
inline bool flush ()
{
// If there are no un-flushed items, do nothing.
if (w == &queue.back ())
return true;
// Try to set 'c' to 'back'
if (c.cas (w, &queue.back ()) != w) {
// Compare-and-swap was unseccessful because 'c' is NULL.
// This means that the reader is asleep. Therefore we don't
// care about thread-safeness and update c in non-atomic
// manner. We'll return false to let the caller know
// that reader is sleeping.
c.set (&queue.back ());
w = &queue.back ();
return false;
}
// Reader is alive. Nothing special to do now. Just move
// the 'first un-flushed item' pointer to the end of the queue.
w = &queue.back ();
return true;
}
// Reads an item from the pipe. Returns false if there is no value.
// available.
inline bool read (T *value_)
{
// Was the value prefetched already? If so, return it.
if (&queue.front () != r) {
*value_ = queue.front ();
queue.pop ();
return true;
}
// There's no prefetched value, so let us prefetch more values.
// (Note that D is a template parameter. Becaue of that one of
// the following branches will be completely optimised away
// by the compiler.)
if (D) {
// If one prefetch was already done since last sleeping,
// don't do a new one, rather ask caller to go asleep.
if (stop) {
stop = false;
return false;
}
// Get new items. Perform the operation in atomic fashion.
r = c.xchg (NULL);
// If there are no elements prefetched, exit and go asleep.
// During pipe's lifetime r should never be NULL, however,
// during pipe shutdown when retrieving messages from it
// to deallocate them, this can happen.
if (&queue.front () == r || !r) {
stop = false;
return false;
}
else {
// We want to do only a single prefetch in D scenario
// before going asleep. Thus, we set stop variable to true
// so that we can return false next time the prefetch is
// attempted.
stop = true;
}
}
else {
// Prefetching in non-D scenario is to simply retrieve the
// pointer from c in atomic fashion. If there are no
// items to prefetch, set c to NULL (using compare-and-swap).
r = c.cas (&queue.front (), NULL);
// If there are no elements prefetched, exit.
// During pipe's lifetime r should never be NULL, however,
// it can happen during pipe shutdown when messages
// are being deallocated.
if (&queue.front () == r || !r)
return false;
}
// There was at least one value prefetched.
// Return it to the caller.
*value_ = queue.front ();
queue.pop ();
return true;
}
protected:
// Allocation-efficient queue to store pipe items.
// Front of the queue points to the first prefetched item, back of
// the pipe points to last un-flushed item. Front is used only by
// reader thread, while back is used only by writer thread.
yqueue_t <T, N> queue;
// Points to the first un-flushed item. This variable is used
// exclusively by writer thread.
T *w;
// Points to the first un-prefetched item. This variable is used
// exclusively by reader thread.
T *r;
// The single point of contention between writer and reader thread.
// Points past the last flushed item. If it is NULL,
// reader is asleep. This pointer should be always accessed using
// atomic operations.
atomic_ptr_t <T> c;
// Used only if 'D' template parameter is set to true. If true,
// prefetch was already done since last sleeping and the reader
// should go asleep instead of prefetching once more.
bool stop;
// Disable copying of ypipe object.
ypipe_t (const ypipe_t&);
void operator = (const ypipe_t&);
};
}
#endif
|
