Do python thread wait for standard output?

if you run couple of threads but they all have to print 开发者_JAVA技巧to the same stdout, does this mean they have to wait on each other? so say if all 4 threads have something to write, they have to pause and wait for the stdout to be free so they can get on with their work?

Deep deep (deep deep deep...) down in the OS's system calls, yes. Modern OSes have thread-safe terminal printing routines which usually just lock around the critical sections that do the actual device access (or buffer, depending on what you're writing into and what its settings are). These waits are very short, however. Keep in mind that this is IO you're dealing with here, so the wait times are likely to be negligible relatively to actual IO execution.

It depends. If stdout is a pipe, each pipe gets a 4KB buffer which you can override when the pipe is created. Buffers are flushed when the buffer is full or with a call to flush().

If stdout is a terminal, output is usually line buffered. So until you print a newline, all threads can write to their buffers. When the newline is written, the whole buffer is dumped on the console and all other threads that are writing newlines at the same time have to wait.

Since threads do other things than writing newlines, each thread gets some CPU. So even in the worst case, the congestion should be pretty small.

There is one exception, though: If you write a lot of data or if the console is slow (like the Linux kernel debug console which uses the serial port). When the console can't cope with the amount of data, more and more threads will hang in the write of the newline waiting for the buffers to flush.