I recently discovered that Linux/OS X provide an important API (TCP_NOTSENT_LOWAT) that lets applications avoid queuing up excessive data inside the kernel's socket send buffers. (The socket send buffers are generally too big, for various reasons.) Unfortunately, it turns out that this API works by controlling when a socket is marked writeable by select and friends, but does not affect whether a send call will succeed, so while you might think these are the same thing they actually aren't. [Edit: it turns out that this description is actually incorrect on Linux, though probably true on macOS -- see] I initially filed a bug on curio about this because curio was assuming they were the same, so I won't repeat all the details: dabeaz/curio#83

@dabeaz points out that asyncio seems to make the same invalid optimization, so filing a bug here too.