1`folly/ThreadCachedInt.h`
2----------------------
3
4High-performance atomic increment using thread caching.
5
6`folly/ThreadCachedInt.h` introduces a integer class designed for high
7performance increments from multiple threads simultaneously without
8loss of precision.  It has two read modes, `readFast` gives a potentially stale
9value with one load, and `readFull` gives the exact value, but is much slower,
10as discussed below.
11
12
13### Performance
14***
15
16Increment performance is up to 10x greater than `std::atomic_fetch_add` in high
17contention environments.  See `folly/test/ThreadCachedIntTest.h` for more
18comprehensive benchmarks.
19
20`readFast` is as fast as a single load.
21
22`readFull`, on the other hand, requires acquiring a mutex and iterating through
23a list to accumulate the values of all the thread local counters, so is
24significantly slower than `readFast`.
25
26
27### Usage
28***
29
30Create an instance and increment it with `increment` or the operator overloads.
31Read the value with `readFast` for quick, potentially stale data, or `readFull`
32for a more expensive but precise result. There are additional convenience
33functions as well, such as `set`.
34
35``` Cpp
36    ThreadCachedInt<int64_t> val;
37    EXPECT_EQ(0, val.readFast());
38    ++val;                        // increment in thread local counter only
39    EXPECT_EQ(0, val.readFast()); // increment has not been flushed
40    EXPECT_EQ(1, val.readFull()); // accumulates all thread local counters
41    val.set(2);
42    EXPECT_EQ(2, val.readFast());
43    EXPECT_EQ(2, val.readFull());
44```
45
46### Implementation
47***
48
49`folly::ThreadCachedInt` uses `folly::ThreadLocal` to store thread specific
50objects that each have a local counter.  When incrementing, the thread local
51instance is incremented.  If the local counter passes the cache size, the value
52is flushed to the global counter with an atomic increment.  It is this global
53counter that is read with `readFast` via a simple load, but will not count any
54of the updates that haven't been flushed.
55
56In order to read the exact value, `ThreadCachedInt` uses the extended
57`readAllThreads()` API of `folly::ThreadLocal` to iterate through all the
58references to all the associated thread local object instances.  This currently
59requires acquiring a global mutex and iterating through the references,
60accumulating the counters along with the global counter.  This also means that
61the first use of the object from a new thread will acquire the mutex in order to
62insert the thread local reference into the list.  By default, there is one
63global mutex per integer type used in `ThreadCachedInt`.  If you plan on using a
64lot of `ThreadCachedInt`s in your application, considering breaking up the
65global mutex by introducing additional `Tag` template parameters.
66
67`set` simply sets the global counter value, and marks all the thread local
68instances as needing to be reset.  When iterating with `readFull`, thread local
69counters that have been marked as reset are skipped.  When incrementing, thread
70local counters marked for reset are set to zero and unmarked for reset.
71
72Upon destruction, thread local counters are flushed to the parent so that counts
73are not lost after increments in temporary threads.  This requires grabbing the
74global mutex to make sure the parent itself wasn't destroyed in another thread
75already.
76
77### Alternate Implementations
78***
79
80There are of course many ways to skin a cat, and you may notice there is a
81partial alternate implementation in `folly/test/ThreadCachedIntTest.cpp` that
82provides similar performance.  `ShardedAtomicInt` simply uses an array of
83`std::atomic<int64_t>`'s and hashes threads across them to do low-contention
84atomic increments, and `readFull` just sums up all the ints.
85
86This sounds great, but in order to get the contention low enough to get similar
87performance as ThreadCachedInt with 24 threads, `ShardedAtomicInt` needs about
882000 ints to hash across.  This uses about 20x more memory, and the lock-free
89`readFull` has to sum up all 2048 ints, which ends up being a about 50x slower
90than `ThreadCachedInt` in low contention situations, which is hopefully the
91common case since it's designed for high-write, low read access patterns.
92Performance of `readFull` is about the same speed as `ThreadCachedInt` in high
93contention environments.
94
95Depending on the operating conditions, it may make more sense to use one
96implementation over the other.  For example, a lower contention environment will
97probably be able to use a `ShardedAtomicInt` with a much smaller array without
98hurting performance, while improving memory consumption and perf of `readFull`.
99