The number of call threads depends on the input parameter max_calls which specifies the maximum number of concurrent RPC calls that can be handled by the server.
调用线程的数量取决于输入参数 max_calls，这个参数指定服务器可以处理的最大并发RPC调用数量。
An asynchronous method may also support multiple pending calls (multiple concurrent invocations), allowing your code to call it any number of times before it completes other pending operations.
异步方法可能还支持多个挂起的调用(多个并行调用)，允许您的代码在此方法完成其他挂起的操作之前调用此方法任意多次。
If the call is made from any other thread, this function returns the default concurrent queue.
如果不在主线程中调用，则返回一个默认的并行队列。
If the call is made from any other thread, this function returns the default concurrent queue.
如果不在主线程中调用，则返回一个默认的并行队列。
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