- #Things for mac synchronize update
- #Things for mac synchronize full
- #Things for mac synchronize code
If the variable is visible from another thread however, such an optimization might prevent the other thread from noticing any changes to it. For local variables, this is usually not a problem.
#Things for mac synchronize code
The compiler often optimizes code by loading the values for variables into registers. Volatile variables apply another type of memory constraint to individual variables. (For an example, see the Wikipedia entry for memory barriers.) To employ a memory barrier, you simply call the OSMemoryBarrier function at the appropriate point in your code. The lack of a memory barrier in such a situation might allow other threads to see seemingly impossible results. Memory barriers are typically used to ensure that memory operations by one thread (but visible to another) always occur in an expected order. A memory barrier acts like a fence, forcing the processor to complete any load and store operations positioned in front of the barrier before it is allowed to perform load and store operations positioned after the barrier.
#Things for mac synchronize update
If seemingly separate variables actually influence each other, the compiler optimizations could update those variables in the wrong order, generating potentially incorrect results.Ī memory barrier is a type of nonblocking synchronization tool used to ensure that memory operations occur in the correct order. Unfortunately, it is not always possible for the compiler to detect all memory-dependent operations. As part of this optimization, the compiler may reorder instructions that access main memory when it thinks doing so would not generate incorrect data.
#Things for mac synchronize full
In order to achieve optimal performance, compilers often reorder assembly-level instructions to keep the instruction pipeline for the processor as full as possible. For a list of supported atomic operations, see the /usr/include/libkern/OSAtomic.h header file or see the atomic man page. Among these operations are atomic versions of the compare-and-swap, test-and-set, and test-and-clear operations. OS X and iOS include numerous operations to perform basic mathematical and logical operations on 32-bit and 64-bit values. For simple operations, such as incrementing a counter variable, this can lead to much better performance than taking a lock. The advantage of atomic operations is that they do not block competing threads. Atomic OperationsĪtomic operations are a simple form of synchronization that work on simple data types. The following sections describe the basic categories of synchronization tools available for you to use. Although avoiding synchronization issues altogether is preferable, it is not always possible. To prevent different threads from changing data unexpectedly, you can either design your application to not have synchronization issues or you can use synchronization tools.
The following sections describe these tools and how you use them in your code to affect safe access to your program’s resources. OS X and iOS provide numerous synchronization tools for you to use, ranging from tools that provide mutually exclusive access to those that sequence events correctly in your application. In cases where your threads must interact, you need to use synchronization tools to ensure that when they interact, they do so safely. A completely interference-free design is not always possible, however. Avoiding shared resources and minimizing the interactions between your threads makes it less likely for those threads to interfere with each other. When it comes to thread safety, a good design is the best protection you have. If you are unlucky, however, the corruption may cause subtle errors that do not manifest themselves until much later, or the errors might require a significant overhaul of your underlying coding assumptions. If you are lucky, the corrupted resource might cause obvious performance problems or crashes that are relatively easy to track down and fix. For example, one thread might overwrite another’s changes or put the application into an unknown and potentially invalid state. Two threads modifying the same resource might interfere with each other in unintended ways. The presence of multiple threads in an application opens up potential issues regarding safe access to resources from multiple threads of execution.
0 kommentar(er)
