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

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Date:	Thu, 20 Feb 2014 17:06:08 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Grant Edwards <grant.b.edwards@...il.com>
CC:	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-rt-users@...r.kernel.org,
	Hal Murray <murray+fedora@...64-139-1-69.sjc.megapath.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Stanislaw Gruszka <sgruszka@...hat.com>
Subject: Re: locking changes in tty broke low latency feature

On 02/20/2014 02:33 PM, Grant Edwards wrote:
> On 2014-02-20, Peter Hurley <peter@...leysoftware.com> wrote:
>> Sender completes 2000 loops in 160ms total run time;
>> that's 80us average per complete round-trip.
>
> If I understand correctly, that 80us _includes_ the actual time for
> the bits on the wire (which means the actual "baud rate" involved is
> high enough that it's negligible).

Yes, 80us includes the transmit time.

>> I think this shows that low_latency is unnecessary and should
>> just be removed/ignored by the tty core.
>
> If that's the sort of latency that you get for typical kernel
> configurations for typical distros, then I agree that the low_latency
> flag is not needed by the tty later.

Stock ubuntu kernel config but preempt and 250hz (and debugging stuff).

> However, it might still be useful for the lower-level tty or
> serial-core driver to control CPU usage vs. latency trade-offs (for
> exaple, one of my drivers uses it to decide where to set the rx FIFO
> threshold).

Sure, it could be left for driver consumption.

Regards,
Peter Hurley

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