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Date:      Tue, 24 Oct 1995 16:34:39 -0400
From:      dennis@etinc.com (dennis)
To:        Bruce Evans <bde@zeta.org.au>
Cc:        hackers@freebsd.org
Subject:   Re: Async utilization.....
Message-ID:  <199510242034.QAA01095@etinc.com>

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>> Its not a real measurement, so you can't use it. Period.
>
>It is a real measurement.
>
>>Figure out the processing requirement for handling one average frame size of 
>> bytes with a 16450 with 8-bit I/O cycles and loads of interrupts, add 20%
>>and compare 
>
>I do that routinely, but it's not as accurate as an actual measurement.
>E.g., typical 16550 input on a 486DX2/66 8MHz ISA bus (non-multiport case),
>for a typical receiver interrupt:
>
>	interrupt overhead					5-10 us
>	i/o's for line status	14
>	i/o's for data		14
>	i/o's for modem status	 1
>	i/o's for interrupt id	 1
>	total i/o's		30 @ 1.25 us each		37.5 us
>								-------
>	h/w dominated i/o overhead				3-3.5 us/byte
>								-----------
>	measured total overhead					6.3 us/byte
>
>For a typical 16550 transmitter interrupt:
>	interrupt overhead					5-10 us
>	i/o's for line status	1
>	i/o's for data		16
>	i/o's for modem status	1
>	i/o's for interrupt id	1
>	total i/o's		19 @ 1.25 us each		24 us
>								-------
>	h/w dominated i/o overhead				1.8-2.1 us/byte
>								-----------
>	measured total overhead					2.9 us/byte
>
>For typical 16450 input:
>
>	interrupt overhead					5-10 us
>	i/o's for line status	 1
>	i/o's for data		 1
>	i/o's for modem status	 1
>	i/o's for interrupt id	 1
>	total i/o's		 4 @ 1.25 us each		 5 us
>								-------
>	h/w dominated i/o overhead				10-15 us/byte
>								-----------
>	measured total overhead					16 us/byte
>
>For WB8013EBT input, per 1500 byte packet:
>
>	interrupt overhead					low
>	i/o's for interrupt	3				low
>	memory mapped i/o's for data	750 @ 0.625 us each
>	other things I forgot					low?
>								-------------
>								more than
>	h/w dominated i/o overhead				0.3125 us/byte
>								-------------
>	measured total overhead					0.7 us/byte
>
>>it  to a single interrupt and one 16-bit bus transfer per frame. It doesn't
>>take a rocket scientist 
>>to know there's a signficant difference in processing requirements.
>
>The factor tends to get reduced a lot by non-data i/o's and protocol
>protocol processing.

This is way too much to have to figure out.....I was talking much more
philosophically......
So much of the processing is in software....aside from the hardware aspect
you have the additional 
tasks for an async implementation such as frame assembly, manual checksum
(OK tables are fast I guess) and
async PPP has more steps than sync ppp because of character mapping and
such. I'm not sure why you're arguing
since no-one really disagrees that async is faster or more efficient...the
bottom line is price.

Dennis
----------------------------------------------------------------------------
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Synchronous Communications Cards and Routers For
Discriminating Tastes. 56k to T1 and beyond. Frame
Relay, PPP, HDLC, and X.25




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