From owner-cvs-all  Sat Dec 22 19:41:24 2001
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From: Bruce Evans <bde@zeta.org.au>
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To: David Greenman <dg@root.com>
Cc: Mike Silbersack <silby@silby.com>,
	Matt Dillon <dillon@FreeBSD.org>, <cvs-committers@FreeBSD.org>,
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Subject: Re: cvs commit: src/sys/dev/sio sio.c
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On Sat, 22 Dec 2001, David Greenman wrote:

> >The driver used to have dynamic fifo trigger reduction, mainly to
> >support many active sio devices, but this was found harmful and backed
> >out:
> >
> >! RCS file: /home/ncvs/src/sys/dev/sio/sio.c,v
> >! Working file: sio.c
> >! head: 1.353
> >! ...
> >! ----------------------------
> >! revision 1.53
> >! date: 1994/09/21 19:39:25;  author: davidg;  state: Exp;  lines: +3 -1
> >! #if 0'd evil dynamic fifo trigger level adjustment; it just bit me and
> >! a couple of other people again.
> >! ----------------------------
>
>    You know, it's really weird to see stuff that I wrote from more than
> 7 years ago. I can only imagine how Kirk feels when he sees log messages
> from the 80's. :-)

At least if they have been untouched for that long then they were probably
correct :-).

>    As I recall, the dynamic adjustment increased interrupt overhead each
> time it ratcheted down, causing it to be even more likely to lower it
> further until it was "1", disabling the overhead reduction benefits of
> the fifo completely. The cure became worse than the disease.

I once planned to bring this back, at least as an option, with the
ratcheting down limited to 1 step.  I never got around to this because
the problem seemed to have gone away except for unusual hardware
combinations.

Bruce


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