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

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Message-ID: <le5laq$40s$1@ger.gmane.org>
Date:	Thu, 20 Feb 2014 19:33:46 +0000 (UTC)
From:	Grant Edwards <grant.b.edwards@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-serial@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: locking changes in tty broke low latency feature

On 2014-02-20, Peter Hurley <peter@...leysoftware.com> wrote:
> On 02/19/2014 09:55 PM, Peter Hurley wrote:
>> On 02/19/2014 06:06 PM, Hal Murray wrote:
>>>> Can you give me an idea of your device's average and minimum required
>>>> latency (please be specific)?  Is your target arch x86 [so I can evaluate the
>>>> the impact of bus-locked instructions relative to your expected]?
>>>
>>> The code I'm familiar with is ntpd and gpsd.  They run on almost any hardware
>>> or OS and talk to a wide collection of devices.
>>>
>>> There is no hard requirement for latency.  They just work better with lower
>>> latency.  The lower the better.
>>>
>>> People gripe about the latency due to USB polling which is about a ms.
>>
>> Have you tried 3.12+ without low_latency? I ripped out a lot of locks
>> from 3.12+ so it's possible it already meets your requirements.
>
> Using Alan's idea to mock up a latency test, I threw together a test jig
> using two computers running 3.14-rc1 and my fwserial driver (modified to
> not aggregrate writes) in raw mode where the target does this:
>
>          while (1) {
>                  read 64 bytes
>                  compare to pattern
>                  write 1 byte response
>          }
>
> and the sender does this:
>
>          for (i = 0; i < 2000; i++) {
>                  write 64-byte pattern
>                  read 1 byte response
>          }
>
> 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).


> 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.

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).

-- 
Grant Edwards               grant.b.edwards        Yow! I wonder if I could
                                  at               ever get started in the
                              gmail.com            credit world?

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