linux-kernel - Re: locking changes in tty broke low latency feature

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Message-ID: <20140219191717.486ac4d0@alan.etchedpixels.co.uk>
Date:	Wed, 19 Feb 2014 19:17:17 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Grant Edwards <grant.b.edwards@...il.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: locking changes in tty broke low latency feature

> How can the requirement be for both must-handle-in-minimum-time data
> (low_latency) and the-userspace-reader-isn't-reading-fast-enough-
> so-its-ok-to-halt-transmission ?

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 first I'd like some hard data on whether or not a low latency
> mode is even necessary (at least for user-space).

The easy way to simulate the annoying as crap worst cases from dumbass
firmware downloaders and the like is to set up a link between two PCs and
time 2000+ repetitions of

send 64 bytes
wait for a Y
send 64 bytes
wait for a Y
....

and the matching far end being a box running an existing kernel or a PIC
or something doing the responses.

Historically we used to lose about 20mS per cycle which over 2000 got to
be a bit of a PITA.

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.

Alan

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