Reads are dreadfully slow on Windows due to improper timeout configuration

[parasyte]

I spent the last few days trying to diagnose why reading 2 MiB over a USB serial port took just over 2 minutes on Windows (compared to 7.3 seconds on macOS for the same device and workload). 02:04.765 on my last test, in fact.

While troubleshooting, I discovered that the GetQueuedCompletionStatusEx call in mio takes 30 ms on average to return any data for a read, regardless of number of bytes returned.

It turns out the problem lies in the way mio-serial overrides the timeouts configured by serialport:

mio-serial/src/lib.rs

Lines 809 to 826 in 49b1c56

	pub(crate) fn override_comm_timeouts(handle: RawHandle) -> StdIoResult<()> {
	let mut timeouts = COMMTIMEOUTS {
	// wait at most 1ms between two bytes (0 means no timeout)
	ReadIntervalTimeout: 1,
	// disable "total" timeout to wait at least 1 byte forever
	ReadTotalTimeoutMultiplier: 0,
	ReadTotalTimeoutConstant: 0,
	// write timeouts are just copied from serialport-rs
	WriteTotalTimeoutMultiplier: 0,
	WriteTotalTimeoutConstant: 0,
	};
	let r = unsafe { SetCommTimeouts(handle, &mut timeouts) };
	if r == 0 {
	return Err(io::Error::last_os_error());
	}
	Ok(())
	}

Or at least, fixing the long 30 ms delays is a matter of commenting all of that out. The complete 2 MiB read process completes on Windows in about 7.8 seconds, on par with macOS.

A side effect of removing this override is that I get spurious 0-byte reads. But I honestly prefer that to 1,600x slower reads! The cost of checking if the length is zero is basically free in comparison.

I'm patching this out in my local builds, but I don't know what you want to do about this ... feature ... for the crate. Making it configurable might be an OK solution, but then I would have to teach tokio-serial to configure it properly. IMHO, ditch this override thing completely.

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Relevant discussions:

• Set Windows timeouts to enforce non-blocking read serialport/serialport-rs#79
• bufReader read_until not handled correctly on Windows serialport/serialport-rs#73 (comment)
• Set Windows timeouts to enforce non-blocking read #38

Quoting the latter comment, "extremely cursed" sounds pretty accurate.