sysprog21 / threadkit

A collection of lightweight threading utilities

ThreadKit: A collection of lightweight threading utilities

This package ThreadKit contains the following threading utilities:

1. Thread pool: simple and usable thread pool.
2. Thread tracer: Lightweight inline thread profiler.
3. Skinny mutex: Low-memory-footprint mutexes for POSIX Threads.
4. Tasklet: Very lightweight thread without its own stack.

Thread pool

Currently, this thread pool implementation

• Works with pthreads only, but API is intentionally opaque to allow other implementations
• Starts all threads on creation of the thread pool.
• Reserves one task for signaling the queue is full.
• Stops and joins all worker threads on destroy.

Possible enhancements

Allow some additional options:

• Lazy creation of threads
• Reduce number of threads automatically
• Unlimited queue size
• Kill worker threads on destroy
• Reduce locking contention

ThreadTracer

ThreadTracer is a lightweight inline profiler that measures wall-time, cpu-time and premptive context switches for threads.

Features

ThreadTracer is an inline profiler that is special in the following ways:

• Fully supports multi threaded applications.
• Will never cause your thread to go to sleep because of profiling.
• Will not miss events.
• Will detect if threads were context-switched by scheduler, preemptively or voluntarily.
• Computes duty-cycle for each scope: not just how long it ran, but also how much of that time, it was scheduled on a core.
• Small light weight system, written in C. Just one header and one small implementation file.
• Zero dependencies.

Limitations

• Doesn't show a live profile, but creates a report after the run, viewable with Google Chrome.
• Currently does not support asynchronous events that start on one thread, and finish on another.

Usage

#include "threadtracer.h"

// Each thread that will be generating profiling events needs to be made known to the system.
TT_ENTRY();

// C Programs need to wrap sections of code with a begin and end macro.
TT_BEGIN("simulation");
simulate( dt );
TT_END("simulation");

// When you are done profiling, typically at program end, or earlier, you can generate the profile report.
TT_REPORT();

Viewing the report

Start the Google Chrome browser, and in the URL bar, type chrome://tracing and then load the genererated threadtracer*.json file.

Note that for the highlighted task, the detail view shows that the thread got interrupted once preemptively, which causes it to run on a CPU core for only 81% of the time that the task took to complete.

The shading of the time slices shows the duty cycle: how much of the time was spend running on a core.

Skipping samples at launch.

To avoid recording samples right after launch, you can skip the first seconds of recording with an environment variable. To skip the first five seconds, do:

$ THREADTRACERSKIP=5 ./foo
ThreadTracer: clock resolution: 1 nsec.
ThreadTracer: skipping the first 5 seconds before recording.
ThreadTracer: Wrote 51780 events (6 discarded) to threadtracer.json

Reference

• chrome://tracing for their excellent in-browser visualization.

Skinny mutex

The main kind of lock provided by the pthreads API is the mutex (pthread_mutex_t). These have a lot of features (enabled though the attributes set in pthread_mutexattr_t), integrate with condition variables, and handle contention gracefully.

But a drawback is their size. On Linux, a pthread_mutex_t occupies 64 bytes on 64-bit machines. If the mutex is protecting a small data structure, this can lead to unwelcome overheads in memory usage, and reduce the effectiveness of caches.

Some pthreads implementations also have spinlocks (pthread_spinlock_t). These are smaller (4 bytes on Linux). But they don't handle contention gracefully, so they are best used for critical sections containing small amounts of code that can be verified to have a short bounded running time.

Hence skinny mutexes provide mutexes that occupy one pointer-sized word. Like pthreads mutexes, they integrate with condition variables and handle contention gracefully, so code using pthreads mutexes can be easily converted to use skinny mutexes instead.

Skinny mutexes use atomic operations to when possible (e.g. when locking or unlocking an uncontended skinny mutex), and fall back to the pthreads primitives when necessary (e.g. when a lock is contended causing a thread to block). So you will still need to compile with -pthread. Performance should generally be similar to pthreads mutexes, and it might even be better in some cases.

Pthread	Skinny mutex
pthread_mutex_t	skinny_mutex_t
pthread_mutex_init	skinny_mutex_init
pthread_mutex_destroy	skinny_mutex_destroy
pthread_mutex_lock	skinny_mutex_lock
pthread_mutex_unlock	skinny_mutex_unlock
pthread_mutex_trylock	skinny_mutex_trylock
pthread_cond_wait	skinny_mutex_cond_wait
pthread_cond_timedwait	skinny_mutex_cond_timedwait
PTHREAD_MUTEX_INITIALIZER	SKINNY_MUTEX_INITIALIZER

Note that skinny_mutex_init does not take an attributes argument (see below for more details). Other than that, all the arguments of the functions mentioned correspond to the pthreads ones, and their specifications and return values are intended to correspond exactly.

In particular, skinny_mutex_lock is not a thread cancellation point, and skinny_mutex_cond_wait is.

Limitations compared to pthread_mutex

Unlike pthreads mutexes, skinny mutexes do not currently support any mutex attributes. Their behavior corresponds to the default pthread mutex attributes (i.e. with NULL passed as the second argument to pthread_mutex_init).

It is possible to add support for error checking corresponding to the PTHREAD_MUTEX_ERRORCHECK type attribute (from pthread_mutexattr_settype). This will probably be a compile-time option.

It seems feasible to add support for the protocol attribute (PTHREAD_PRIO_INHERIT and PTHREAD_PRIO_PROTECT from pthread_mutexattr_setprotocol). There might be room for improvements.

The PTHREAD_MUTEX_RECURSIVE type attribute will not be supported, as it would require skinny_mutex_t to grow, and you can rewrite code to avoid the need for recursive mutexes.

Support for the process-shared and priority ceiling attributes (pthread_mutexattr_setpshared and pthread_mutexattr_setprioceiling) is also unlikely, as they seem to be of marginal usefulness and/or hard to implement.

Tasklet

A tasklet is a sequential context of execution. Like a thread, a tasklet can wait for events (such as data arriving on a socket). Unlike a thread, a tasklet does not have its own stack, so tasklet code has to be follow certain idioms. But those idioms are less cumbersome than trying to write callback-based code in C, particularly in a multithreaded context.

Tasklets are very lightweight; many millions of tasklets could fit in the memory of a modern machine. A scalable service can schedule runnable tasklets onto a much smaller number of threads.