Notes on Numba’s threading implementation

The execution of the work presented by the Numba parallel targets is undertaken by the Numba threading layer. Practically, the “threading layer” is a Numba built-in library that can perform the required concurrent execution. At the time of writing there are three threading layers available, each implemented via a different lower level native threading library. More information on the threading layers and appropriate selection of a threading layer for a given application/system can be found in the threading layer documentation.

The pertinent information to note for the following sections is that the function in the threading library that performs the parallel execution is the parallel_for function. The job of this function is to both orchestrate and execute the parallel tasks.

The relevant source files referenced in this document are

  • numba/np/ufunc/tbbpool.cpp

  • numba/np/ufunc/omppool.cpp

  • numba/np/ufunc/workqueue.c

    These files contain the TBB, OpenMP, and workqueue threadpool implementations, respectively. Each includes the functions set_num_threads(), get_num_threads(), and get_thread_id(), as well as the relevant logic for thread masking in their respective schedulers. Note that the basic thread local variable logic is duplicated in each of these files, and not shared between them.

  • numba/np/ufunc/parallel.py

    This file contains the Python and JIT compatible wrappers for set_num_threads(), get_num_threads(), and get_thread_id(), as well as the code that loads the above libraries into Python and launches the threadpool.

  • numba/parfors/parfor_lowering.py

    This file contains the main logic for generating code for the parallel backend. The thread mask is accessed in this file in the code that generates scheduler code, and passed to the relevant backend scheduler function (see below).

Thread masking

As part of its design, Numba never launches new threads beyond the threads that are launched initially with numba.np.ufunc.parallel._launch_threads() when the first parallel execution is run. This is due to the way threads were already implemented in Numba prior to thread masking being implemented. This restriction was kept to keep the design simple, although it could be removed in the future. Consequently, it’s possible to programmatically set the number of threads, but only to less than or equal to the total number that have already been launched. This is done by “masking” out unused threads, causing them to do no work. For example, on a 16 core machine, if the user were to call set_num_threads(4), Numba would always have 16 threads present, but 12 of them would sit idle for parallel computations. A further call to set_num_threads(16) would cause those same threads to do work in later computations.

Thread masking was added to make it possible for a user to programmatically alter the number of threads performing work in the threading layer. Thread masking proved challenging to implement as it required the development of a programming model that is suitable for users, easy to reason about, and could be implemented safely, with consistent behavior across the various threading layers.

Programming model

The programming model chosen is similar to that found in OpenMP. The reasons for this choice were that it is familiar to a lot of users, restricted in scope and also simple. The number of threads in use is specified by calling set_num_threads and the number of threads in use can be queried by calling get_num_threads.These two functions are synonymous with their OpenMP counterparts (with the above restriction that the mask must be less than or equal to the number of launched threads). The execution semantics are also similar to OpenMP in that once a parallel region is launched, altering the thread mask has no impact on the currently executing region, but will have an impact on parallel regions executed subsequently.

The Implementation

So as to place no further restrictions on user code other than those that already existed in the threading layer libraries, careful consideration of the design of thread masking was required. The “thread mask” cannot be stored in a global value as concurrent use of the threading layer may result in classic forms of race conditions on the value itself. Numerous designs were discussed involving various types of mutex on such a global value, all of which were eventually broken through thought experiment alone. It eventually transpired that, following some OpenMP implementations, the “thread mask” is best implemented as a thread local. This means each thread that executes a Numba parallel function will have a thread local storage (TLS) slot that contains the value of the thread mask to use when scheduling threads in the parallel_for function.

The above notion of TLS use for a thread mask is relatively easy to implement, get_num_threads and set_num_threads simply need to address the TLS slot in a given threading layer. This also means that the execution schedule for a parallel region can be derived from a run time call to get_num_threads. This is achieved via a well known and relatively easy to implement pattern of a C library function registration and wrapping it in the internal Numba implementation.

In addition to satisfying the original upfront thread masking requirements, a few more complicated scenarios needed consideration as follows.

Nested parallelism

In all threading layers a “main thread” will invoke the parallel_for function and then in the parallel region, depending on the threading layer, some number of additional threads will assist in doing the actual work. If the work contains a call to another parallel function (i.e. nested parallelism) it is necessary for the thread making the call to know what the “thread mask” of the main thread is so that it can propagate it into the parallel_for call it makes when executing the nested parallel function. The implementation of this behavior is threading layer specific but the general principle is for the “main thread” to always “send” the value of the thread mask from its TLS slot to all threads in the threading layer that are active in the parallel region. These active threads then update their TLS slots with this value prior to performing any work. The net result of this implementation detail is that:

  • thread masks correctly propagate into nested functions

  • it’s still possible for each thread in a parallel region to safely have a different mask with which to call nested functions, if it’s not set explicitly then the inherited mask from the “main thread” is used

  • threading layers which have dynamic scheduling with threads potentially joining and leaving the active pool during a parallel_for execution are successfully accommodated

  • any “main thread” thread mask is entirely decoupled from the in-flux nature of the thread masks of the threads in the active thread pool

Python threads independently invoking parallel functions

The threading layer launch sequence is heavily guarded to ensure that the launch is both thread and process safe and run once per process. In a system with numerous Python threading module threads all using Numba, the first thread through the launch sequence will get its thread mask set appropriately, but no further threads can run the launch sequence. This means that other threads will need their initial thread mask set some other way. This is achieved when get_num_threads is called and no thread mask is present, in this case the thread mask will be set to the default. In the implementation, “no thread mask is present” is represented by the value -1 and the “default thread mask” (unset) is represented by the value 0. The implementation also immediately calls set_num_threads(NUMBA_NUM_THREADS) after doing this, so if either -1 or 0 is encountered as a result from get_num_threads() it indicates a bug in the above processes.

OS fork() calls

The use of TLS was also in part driven by the Linux (the most popular platform for Numba use by far) having a fork(2, 3P) call that will do TLS propagation into child processes, see clone(2)’s CLONE_SETTLS.

Thread ID

A private get_thread_id() function was added to each threading backend, which returns a unique ID for each thread. This can be accessed from Python by numba.np.ufunc.parallel._get_thread_id() (it can also be used inside a JIT compiled function). The thread ID function is useful for testing that the thread masking behavior is correct, but it should not be used outside of the tests. For example, one can call set_num_threads(4) and then collect all unique _get_thread_id()s in a parallel region to verify that only 4 threads are run.

Caveats

Some caveats to be aware of when testing thread masking:

  • The TBB backend may choose to schedule fewer than the given mask number of threads. Thus a test such as the one described above may return fewer than 4 unique threads.

  • The workqueue backend is not threadsafe, so attempts to do multithreading nested parallelism with it may result in deadlocks or other undefined behavior. The workqueue backend will raise a SIGABRT signal if it detects nested parallelism.

  • Certain backends may reuse the main thread for computation, but this behavior shouldn’t be relied upon (for instance, if propagating exceptions).

Use in Code Generation

The general pattern for using get_num_threads in code generation is

import llvmlite.llvmpy.core as lc

get_num_threads = builder.module.get_or_insert_function(
    lc.Type.function(lc.Type.int(types.intp.bitwidth), []),
    name="get_num_threads")

num_threads = builder.call(get_num_threads, [])

with cgutils.if_unlikely(builder, builder.icmp_signed('<=', num_threads,
                                              num_threads.type(0))):
    cgutils.printf(builder, "num_threads: %d\n", num_threads)
    context.call_conv.return_user_exc(builder, RuntimeError,
                                              ("Invalid number of threads. "
                                               "This likely indicates a bug in Numba.",))

# Pass num_threads through to the appropriate backend function here

See the code in numba/parfors/parfor_lowering.py.

The guard against num_threads being <= 0 is not strictly necessary, but it can protect against accidentally incorrect behavior in case the thread masking logic contains a bug.

The num_threads variable should be passed through to the appropriate backend function, such as do_scheduling or parallel_for. If it’s used in some way other than passing it through to the backend function, the above considerations should be taken into account to ensure the use of the num_threads variable is safe. It would probably be better to keep such logic in the threading backends, rather than trying to do it in code generation.