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beginning section on atomics

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Perry Kivolowitz 2023-02-28 20:48:44 -06:00
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],
"files.associations": {
"ostream": "cpp",
"iomanip": "cpp"
"iomanip": "cpp",
"atomic": "cpp"
}
}

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# Section 1 / Atomics
![Gurney Hallek](./battle_pug.jpeg)
## Threads
Suppose you run two copies of the same program at the same time. They
both have a variable named `i`. Can a change to `i` made by one copy of
the program impact the value of `i` in the other running copy of the
program? Of course not.
Think of running two copies of a program at the same time as having
two identical, but distinct, homes. What happens inside one house does
not impact what happens inside the house next door.
Threads are a different way of getting more than one "copy" of a program
to run at the same time. Threads are different, however, in that they
all live within the same household. All of the housemates share the
living space and all housemates have access to any global or shared
resource. This makes for great gains in performance for a broad class
of problems but also introduces great hazards.
Suppose you buy a carton of milk and place it in the fridge. If you were
the only member of the household you would expect that when we next went
to the fridge your milk would still be there, right? If you share the
household with other people, this might not be the case.
Consider the following program:
```c++
#include <iostream> // 1
#include <thread> // 2
#include <atomic> // 3
#include <vector> // 4
// 5
using std::cout; // 6
using std::endl; // 7
using std::atomic; // 8
using std::vector; // 9
using std::thread; // 10
// 11
const uint32_t MAX_LOOPS = 10000; // 12
const uint32_t NUM_THREADS = 16; // 13
// 14
/* volatile is necessary if any use of the optimizer // 15
is to be made. // 16
*/ // 17
volatile uint32_t naked_int = 0; // 18
atomic<uint32_t> atomic_integer(0); // 19
// 20
void NakedWorker() { // 21
extern volatile uint32_t naked_int; // 22
// 23
for (uint32_t i = 0; i < MAX_LOOPS; i++) { // 24
naked_int++; // 25
} // 26
} // 27
// 28
void AtomicWorker() { // 29
extern atomic<uint32_t> atomic_integer; // 30
// 31
for (uint32_t i = 0; i < MAX_LOOPS; i++) { // 32
atomic_integer++; // 33
} // 34
} // 35
// 36
void DoNaked() { // 37
vector<thread *> threads; // 38
// 39
for (uint32_t i = 0; i < NUM_THREADS; i++) { // 40
threads.push_back(new thread(NakedWorker)); // 41
} // 42
// 43
for (auto &t : threads) { // 44
t->join(); // 45
} // 46
} // 47
// 48
void DoAtomic() { // 49
vector<thread *> threads; // 50
// 51
for (uint32_t i = 0; i < NUM_THREADS; i++) { // 52
threads.push_back(new thread(AtomicWorker)); // 53
} // 54
// 55
for (auto &t : threads) { // 56
t->join(); // 57
} // 58
} // 59
// 60
int main() { // 61
// 62
DoNaked(); // 63
DoAtomic(); // 64
// 65
cout << "Correct sum is: "; // 66
cout << NUM_THREADS * MAX_LOOPS << endl; // 67
cout << "Naked sum: " << naked_int << endl; // 68
cout << "Atomic sum: " << atomic_integer << endl; // 69
// 70
return 0; // 71
} // 72
perrykivolowitz@DAEDALUS atomics %
```
This program will spawn 16 threads which will each loop 10,000 times,
adding one to a zero-initialized integer each loop. At the end, when all
the threads complete, the integer should have the value 160,000.
Alas, this is an example of the class "Hidden Update" bug. The shared
resource, the integer, will get clobbered in unpredictable ways.
For example, multiple runs might produce:
- Naked sum: 74291
- Naked sum: 79390
- Naked sum: 89115
- etc
## Serializing Access to Integer Types
C++11 introduced the notion of *atomic integers*. These do not glow.
Rather, access to them is guaranteed to be atomic... as in, cannot be
broken down. The hidden update problem's root cause is that adding
(for example) to a value in memory involves three instructions at the
assembly language level. A load, an addition, and a store. The hidden
update occurs when a thread is yanked from the CPU in the middle of
these instructions. When the thread returns to the CPU, the store
causes old data to overwrite (hide) correct data.

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#include <iostream>
#include <thread>
#include <atomic>
#include <vector>
using std::cout;
using std::endl;
using std::atomic;
using std::vector;
using std::thread;
const uint32_t MAX_LOOPS = 10000;
const uint32_t NUM_THREADS = 16;
/* volatile is necessary if any use of the optimizer
is to be made.
*/
volatile uint32_t naked_int = 0;
atomic<uint32_t> atomic_integer(0);
void NakedWorker() {
extern volatile uint32_t naked_int;
for (uint32_t i = 0; i < MAX_LOOPS; i++) {
naked_int++;
}
}
void AtomicWorker() {
extern atomic<uint32_t> atomic_integer;
for (uint32_t i = 0; i < MAX_LOOPS; i++) {
atomic_integer++;
atomic_integer = atomic_integer * 30;
}
}
void DoNaked() {
vector<thread *> threads;
for (uint32_t i = 0; i < NUM_THREADS; i++) {
threads.push_back(new thread(NakedWorker));
}
for (auto &t : threads) {
t->join();
}
}
void DoAtomic() {
vector<thread *> threads;
for (uint32_t i = 0; i < NUM_THREADS; i++) {
threads.push_back(new thread(AtomicWorker));
}
for (auto &t : threads) {
t->join();
}
}
int main() {
DoNaked();
DoAtomic();
cout << "Correct sum is: ";
cout << NUM_THREADS * MAX_LOOPS << endl;
cout << "Naked sum: " << naked_int << endl;
cout << "Atomic sum: " << atomic_integer << endl;
return 0;
}

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