Re: HZ != 1000 causes problem with serial device shown by git-bisect

[Bill Davidsen]

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@xxxxxxx)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me

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