> > Because low latency is about *turn around* time. There are plenty of > > protocols that can flow control, do flow control and want low latency > > because they are not windowed. It's not mutually exclusive by any means. > > But if it's all about turn around time, how can the situation devolve to > needing throttling in the first place? For N_TTY, throttling only happens > when the read queue is close to overflow (only 128 bytes left in 4k buffer). Because if we are on the uplink end of a dumb protocol then our flow ends up as < 4K in read rest of block read block complete write Y until at some point sods law decrees that we do a big load of disk writes, an SMM event steals the processor, firefox garbage collects or somesuch and we go 4K in throttle flow control It only has to happen *once*, not every time to screw stuff up. > If the reader isn't pulling _all_ the data out the instant it's woken, > trying to trim off one worker wakeup (by processing input at interrupt time) > is pointless. No - because of the statistical corner cases. The standard Linux is not remotely hard real time, and even if it were most of the tools involved are not. > > Low latency goes back to the days of flip buffers, bottom halves an a > > 100Hz clock. There are certainly better ways to do it now if its needed. > > Still, as you've pointed out a couple of times, maybe there's a limited > fastpath that's easy and simple. That's why I was asking about throttling > because it's evaluated even for raw mode. Even if there isn't the goal of low latency was always 'get stuff to happen soon' not 'get stuff to happen in the IRQ handler' If you have the tty processing happening immediately after the IRQ returns when low_latency is set I'm sure the numbers will be just fine and as good as historically. Whether you can always do that I guess depends what happens on really slow boxes if you don't do any deferral - eg on Geert's Amiga. Flow control even in raw mode is expected Unix behaviour. However do we ever need to evaluate it if the tty termios has IXON and CRTSCTS clear ? Alan -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
