Details on lock-free buffer management » History » Version 9
Eric Flumerfelt, 10/17/2018 03:00 PM
1 | 1 | Eric Flumerfelt | h1. Details on lock-free buffer management |
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2 | 1 | Eric Flumerfelt | |
3 | 1 | Eric Flumerfelt | The _artdaq_ shared memory interface uses several flags in the buffer descriptor structure to achieve self-contained, lock-free operation. |
4 | 1 | Eric Flumerfelt | |
5 | 1 | Eric Flumerfelt | # std::atomic<BufferSemaphoreFlags> sem |
6 | 1 | Eric Flumerfelt | ** The state of the buffer. One of Empty, Writing, Full, or Reading. |
7 | 1 | Eric Flumerfelt | # std::atomic<int16_t> sem_id |
8 | 9 | Eric Flumerfelt | ** The current owner of the buffer. Each instance of SharedMemoryManager (readers and writers) takes an ID number (from ShmStruct::std::atomic<int> next_id), and uses this number to indicate that it is the current owner of the buffer. Buffers without owners have sem_id set to -1. |
9 | 1 | Eric Flumerfelt | # std::atomic<uint64_t> last_touch_time |
10 | 1 | Eric Flumerfelt | ** The last time (microseconds, system clock) the buffer was "touched". Buffers should be touched whenever a modifying operation occurs, such as writes or reads, or before the buffer transitions from an inactive state (Empty, Full) to an active state (Writing, Reading). |
11 | 2 | Eric Flumerfelt | |
12 | 1 | Eric Flumerfelt | * The checkBuffer_ function is used to determine if the buffer's sem and sem_id are currently set to the expected values. No read or write operations should be performed without calling this function. |
13 | 9 | Eric Flumerfelt | * sem should only be updated by the owner of the buffer, except in the timeout case where another manager (usually ID 0) notices that the buffer has not been touched for longer than the configured timeout interval. |
14 | 6 | Eric Flumerfelt | * Buffer touch time should only be updated by the owner of the buffer, except the case of unowned (sem_id == -1) buffers which can be updated by anyone. Buffers may be touched when in any mode. The only negative side effect of a buffer being touched out-of-turn is that it may take longer for a timeout to occur. |
15 | 1 | Eric Flumerfelt | ** The logic for resetting buffers from an active state to a passive state is in the ResetBuffer function. Additionally, when buffer read operations do not empty the buffer, ResetBuffer ensures that old broadcasts are removed from the shared memory once they have timed out. |
16 | 6 | Eric Flumerfelt | |
17 | 6 | Eric Flumerfelt | h2. Operations on buffers |
18 | 6 | Eric Flumerfelt | |
19 | 6 | Eric Flumerfelt | h3. Acquire a buffer for reading/writing |
20 | 6 | Eric Flumerfelt | |
21 | 7 | Eric Flumerfelt | # Check @sem_id == -1@ |
22 | 6 | Eric Flumerfelt | # Check sem for corresponding inactive state (Empty for writing, Full for reading) |
23 | 6 | Eric Flumerfelt | # Touch buffer |
24 | 6 | Eric Flumerfelt | # Update sem_id |
25 | 7 | Eric Flumerfelt | # Check @sem_id == manager_id_@ |
26 | 6 | Eric Flumerfelt | # Update sem |
27 | 6 | Eric Flumerfelt | # Check sem |
28 | 7 | Eric Flumerfelt | # Check @sem_id == manager_id_@ |
29 | 6 | Eric Flumerfelt | # Touch buffer |
30 | 6 | Eric Flumerfelt | |
31 | 6 | Eric Flumerfelt | h3. Read/Write from buffer |
32 | 6 | Eric Flumerfelt | |
33 | 7 | Eric Flumerfelt | # Check @sem_id == manager_id_@ |
34 | 7 | Eric Flumerfelt | # Check @sem == Reading/Writing@ |
35 | 6 | Eric Flumerfelt | # Touch buffer |
36 | 1 | Eric Flumerfelt | # Read from/Write to buffer |
37 | 1 | Eric Flumerfelt | # Touch buffer |
38 | 1 | Eric Flumerfelt | |
39 | 1 | Eric Flumerfelt | h3. Check buffer for timeout in non-broadcast mode |
40 | 1 | Eric Flumerfelt | |
41 | 7 | Eric Flumerfelt | # Read last_touch_time and calculate delta from current time |
42 | 8 | Eric Flumerfelt | # IF @delta < timeout@ OR @sem == Empty@ *return false* |
43 | 8 | Eric Flumerfelt | # IF @sem_id == manager_id_@ AND @sem == Writing@ *return true* <-- This should never happen! |
44 | 7 | Eric Flumerfelt | # IF @sem_id != manager_id_@ AND @sem == Reading@ THEN |
45 | 7 | Eric Flumerfelt | ## Re-calculate delta |
46 | 8 | Eric Flumerfelt | ## IF @delta < timeout@ *return false* |
47 | 8 | Eric Flumerfelt | ## *return true* |
48 | 8 | Eric Flumerfelt | # *return false* |
49 | 7 | Eric Flumerfelt | |
50 | 1 | Eric Flumerfelt | h3. Check buffer for timeout in broadcast mode |
51 | 7 | Eric Flumerfelt | |
52 | 7 | Eric Flumerfelt | # Read last_touch_time and calculate delta from current time |
53 | 8 | Eric Flumerfelt | # IF @delta < timeout@ OR @sem == Empty@ *return false* |
54 | 8 | Eric Flumerfelt | # IF @sem_id == manager_id_@ AND @sem == Writing@ *return true* <-- This should never happen! |
55 | 7 | Eric Flumerfelt | # IF @sem == Full@ AND (@manager_id_ == 0@ OR buffer already seen) THEN |
56 | 7 | Eric Flumerfelt | ## Reset buffer to Empty state |
57 | 8 | Eric Flumerfelt | ## *return true* |
58 | 7 | Eric Flumerfelt | # IF @sem_id != manager_id_@ AND @sem == Reading@ THEN |
59 | 7 | Eric Flumerfelt | ## Re-calculate delta |
60 | 8 | Eric Flumerfelt | ## IF @delta < timeout@ *return false* |
61 | 7 | Eric Flumerfelt | ## Reset buffer to Full state |
62 | 8 | Eric Flumerfelt | ## *return true* |
63 | 8 | Eric Flumerfelt | # *return false* |
64 | 3 | Eric Flumerfelt | |
65 | 5 | Eric Flumerfelt | !buffer_state_diagram.png! |