Concurency threading models

Mircea Baja - 28 May 2025

# Concurency threading models

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---

# Single threaded (ST)

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---

# Single threaded (ST)

- in a pure single threaded model the advantage is that no thread
  synchronization is required: minimalistic code
- but then not really useful because of the lack of interaction with other
  threads
  - even things like CTRL-C in a Windows console requires reacting to events
    from other threads
  - so external cancellations are practically out of the picture
- maybe useful for simulations that run to completion

- BUT: it makes a great learning tool
  - because of the minimalistic code
  - and can compare overhead of solutions that use thread synchronization

---

# Single threaded (ST) - detail

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---

# Single threaded (ST) - detail

- the "queue" is more complicated if we want to have timers:
  - the ready queue: work that is ready to be executed
  - the timers heap: min heap of timers (the one at the top is the first one to
    expire)
- the thread reads alternatively from the ready queue and the timers heap
- if the ready queue is empty and the next timer has not yet expired then sleep
  until the timer is due to expire
- some fairness logic is required to ensure that it can't get into loops where
  some part of the queue is ignored

---

# Multi-threaded (MT)

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---

# Multi-threaded (MT)

- access to the queue requires synchronization
- more subtly data shared between different streams of work requires
  synchronization
- at least a std::mutex is required to wrap access

---

# Multi-threaded (MT) - detail

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---

# Multi-threaded (MT) - detail

- there is a zoo of options available wait-free/lock-free etc.
- in particular for queues of "ready to do work" that are processed by multiple
  threads you might want to look at least at not using a single `std::mutex`, but
  break the queue into multiple smaller queues (one per thread), each with it's
  own lock and the ability to "steal" work when a thread completes it's own
  work faster than other processing threads see [Better Code: Concurrency -
  Sean Parent](https://www.youtube.com/watch?v=zULU6Hhp42w)

- other threads can add work to the queue
- there can be additional queues e.g. for a "event" that can notify, they would
  also require synchronization

---

# Heterogeneous apartment

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---

# Heterogeneous apartment

- different kind of queues: pool of threads (or thread) blocked reading from queue
- need to schedule execution on different kind of threads that have the
  capability to execute the operation/computation/work/API

- sometimes queues are build in hardware: interrupt priority in embedded
  environments
- sometimes code/computation is executed on GPUs

- a large/complex application might have multiple communicating heterogeneous
  apartments

---

# Mainly one thread (T1)

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---

# Mainly one thread (T1) - example

- `ReadFileEx`: provide C callback (completion routine)
- `SleepEx`: callbacks are called
  - use top of the min heap to determine the sleep duration
- Windows thread pool for Windows events
  - when a Windows event is set, from a thread in the default Windows thread
    pool, add to the "ready queue", signal event, use `WaitForSingleObjectEx`
    instead of `SleepEx`

---

# Mainly one thread (T1) - locking

- can reduce synchronization requirements because only one activity will be run
  at a time

### Many T1 variations

E.g. locking strategies:
- many low granularity locks (e.g. one for the "ready queue", another one for
  the "timer heap"
- single lock: e.g. in the style of the Python GIL (Global Interpreter Lock)

- care needs to be taken for cancellations triggered from another thread
  - one option is to schedule it on the "ready queue" so that the cancellation
    work is done from the "one thread"

---

# Bounded parallelism - IMPORTANT!

- the case with blocking threads
  - you have N threads in a pool
  - one tread waits for a activity to be executed on another thread in the pool
  - now you have N - 1 threads in the pool
- "a multi-threaded program is not correct if it can't run single-threaded"
  - described by Sean Parent when a single thread was used to run application
    in browser

Common blocking to avoid:
- in the destructor
  - don't block in the destructor waiting
- cancellation
  - don't block waiting for work to react to cancellation trigger
- event signalling
  - don't wait for work to be completed on event notification
- std::future.get()
  - inherently blocking (another bad thing about std::future)

---

# Questions?