Message-ID: <442AC46F.2060107@tmr.com>
Date: Wed, 29 Mar 2006 12:31:27 -0500
From: Bill Davidsen <davidsen@tmr.com>
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: linux-kernel@vger.kernel.org, rmk+kernel@arm.linux.org.uk
Subject: Re: HZ != 1000 causes problem with serial device shown by   git-bisect
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Alan Cox wrote:
> On Llu, 2006-03-27 at 18:46 -0500, Greg Lee wrote:
>> I am having a problem when using PPP over a particular PCMCIA based serial device and have
>> pinned the problem down using git-bisect to this particular commit that was made between
>> 2.6.12.6 and 2.6.13:
> 
> That would make sense.
> 
>> I have also tried a number of other kernels and the problem exists all the way to 2.6.15.6
>> but is fixed in 2.6.16, so I am going to git-bisect 2.6.15.6 to 2.6.16, but I thought I
>> would get this message out now in case someone has an inkling of what the problem is.
> 
> I think I can tell you fairly accurately if you are running fairly high
> data rates.
> 
> The old pre 2.6.16 tty code works something like this
> 
> Each serial IRQ
> 	add chars to buffer if they fit
> 
> Each timer IRQ
> 	switch buffers
> 	process original buffer
> 
> 
> So the higher HZ is the faster data speed you can do. With very high
> data rates lower HZ means more dropped characters.
> 
> 2.6.16 implements the new tty layer which replaces this with a proper
> buffering and queueing mechanism and is SMP aware (and thanks to Paul
> rather SMP clever too). 

Just as an aside to this and thanks to Paul, this seems in practice to 
work as well with HT (as I would expect) and handle fairly high rate 
(230kb) connections perfectly. I hope it applies for dumb multiport 
cards as well, I have a fair number of them here and there.

-- 
    -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
  last possible moment - but no longer"  -me

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