The post argues that the term "thread contention" is misused in the context of Ruby's Global VM Lock (GVL). True thread contention involves multiple threads attempting to modify the same shared resource simultaneously. However, in Ruby with the GVL, only one thread can execute Ruby code at any given time. What appears as "contention" is actually just queuing: threads waiting their turn to acquire the GVL. The post emphasizes that understanding this distinction is crucial for profiling and optimizing Ruby applications. Instead of focusing on eliminating "contention," developers should concentrate on reducing the time threads hold the GVL, minimizing the queueing time and improving overall performance.
The blog post "Ruby “Thread Contention” Is Simply GVL Queuing" by Peter Cooper argues that the common phrase "thread contention" is often misused in the context of Ruby's multi-threading performance limitations, leading to confusion and misdiagnosis of performance issues. Instead of actual contention, where multiple threads are actively competing for the same shared resource simultaneously, the performance bottleneck in Ruby's multi-threaded applications typically stems from the Global Virtual Machine Lock (GVL).
Cooper elaborates that the GVL, a core mechanism in the standard implementation of Ruby known as CRuby, serializes execution of Ruby code. While multiple operating system (OS) threads can exist within a Ruby process, only one Ruby thread can execute Ruby bytecode at any given instant due to the GVL. Other threads wishing to execute Ruby code are placed in a queue managed by the GVL. This queuing mechanism is what frequently gets misinterpreted as "thread contention."
The author meticulously distinguishes between true contention and GVL queuing. True contention occurs when multiple threads attempt to access and modify the same shared resource, such as a shared memory location or a file, simultaneously. This leads to race conditions and requires synchronization primitives like mutexes or semaphores to ensure data integrity and prevent unexpected behavior. However, in Ruby, the GVL inherently prevents true contention within Ruby code itself because it restricts execution to a single thread at a time.
Therefore, what appears as "contention" in Ruby profiling tools is actually the overhead of the GVL's queuing mechanism. Threads wait their turn to acquire the GVL, creating a queue of waiting threads. This queuing and dequeuing process, along with the context switching between threads managed by the operating system, contributes to the perceived performance bottleneck. Cooper emphasizes that this is not true contention for resources, but rather a queuing delay imposed by the GVL's serialization of Ruby code execution.
The post further clarifies that true contention can still occur in Ruby in scenarios involving non-Ruby code, particularly when interacting with external libraries or system calls. For example, if multiple Ruby threads simultaneously attempt to write to the same file descriptor, true contention can occur at the operating system level, outside the scope of the GVL.
In summary, Cooper advocates for more precise language when discussing Ruby's multi-threading performance. He argues that using the term "GVL queuing" instead of "thread contention" provides a more accurate description of the performance bottleneck in CRuby, highlighting the role of the GVL and its queuing mechanism as the primary source of performance limitations in multi-threaded Ruby applications and facilitating more effective diagnosis and optimization of such applications.
Summary of Comments ( 38 )
https://news.ycombinator.com/item?id=42916203
HN commenters generally agree with the author's premise that Ruby's "thread contention" is largely a misunderstanding of the GVL (Global VM Lock). Several pointed out that true contention can occur in Ruby, specifically around I/O operations and interactions with native extensions/C code that release the GVL. One commenter shared a detailed example of contention in a Rails app due to database connection pooling. Others highlighted that the article might undersell the performance impact of the GVL, particularly for CPU-bound tasks, where true parallelism is impossible. The real takeaway, according to the comments, is to understand the GVL's limitations and choose the right concurrency model (e.g., processes, async I/O) for the specific task, rather than blindly reaching for threads. Finally, a few commenters discussed the complexities of truly removing the GVL from Ruby, citing the challenges and potential breakage of existing code.
The Hacker News post titled "Ruby “Thread Contention” Is Simply GVL Queuing" has generated several comments discussing the nuances of Ruby's Global VM Lock (GVL) and its impact on concurrency.
One commenter points out the distinction between "true contention" and mere queuing for the GVL. They argue that while multiple threads might appear to be contending for resources, the actual bottleneck is often the serialized execution enforced by the GVL. This commenter further emphasizes that profiling tools might misrepresent this queuing as contention, leading developers to misdiagnose performance issues. They suggest that a more accurate term would be "GVL contention" or "GVL queuing" to reflect the underlying mechanism.
Another commenter concurs, adding that while the GVL doesn't eliminate all forms of contention (e.g., contention for shared memory), it does significantly influence how threads interact with resources. They highlight the importance of understanding this distinction when optimizing Ruby code for multi-threaded environments.
A further comment delves into the complexities of the GVL's implementation, noting that its behavior can vary across different Ruby interpreters (e.g., MRI, JRuby, TruffleRuby). This commenter emphasizes the need to consider the specific interpreter when analyzing GVL-related performance characteristics. They also mention the potential benefits and drawbacks of using alternative concurrency models, such as fibers and actors, in Ruby.
Another discussion thread focuses on the practical implications of the GVL for Ruby developers. Commenters share their experiences with debugging and optimizing multi-threaded Ruby applications, offering advice on how to mitigate the performance limitations imposed by the GVL. Specific techniques, such as using asynchronous I/O operations and carefully managing shared resources, are discussed.
One commenter offers a contrasting perspective, arguing that the term "thread contention" is still relevant in the context of the GVL. They explain that even though the GVL serializes execution, threads are still competing for the opportunity to acquire the lock. This competition, they contend, can still be considered a form of contention, albeit one mediated by the GVL.
Overall, the comments on the Hacker News post provide a rich discussion on the intricacies of the GVL in Ruby. They highlight the importance of understanding the GVL's impact on concurrency, the potential for misinterpreting profiling data, and the strategies developers can employ to optimize their multi-threaded Ruby code. The comments also reveal the ongoing debate about the appropriate terminology for describing the GVL's effects on thread behavior.