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Date: 9 Dec 1995 16:37:11 +0800
From: peter@haywire.dialix.com (Peter Wemm)
Message-ID: <4abhrn$hv8$1@haywire.DIALix.COM>
Organization: DIALix Services, Perth, Australia.
References: <28835.818443536@time.cdrom.com>, <199512081834.LAA00699@rocky.sri.MT.net>
Subject: Re: User space PPP & leased line
Sender: owner-hackers@freebsd.org
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nate@rocky.sri.MT.net (Nate Williams) writes:

>> > One thing we DID notice was that the transfer speeds using the user mode 
>> > code were SIGNIFICANTLY higher than those using the kernel mode code - 
>> > for example, transferring a medium size binary file (one that could be 
>> > fully transferred in < 2 minutes :-) using pppd takes an average of 
>> > 2.3KB/s whereas with ijppp it's more like 3.3KB/s (using a V.fc modem 
>> > connection at 26.4Kb/s).  I'm not sure why this should be, but given the 
>> 
>> That's also worth investigating.  I can't see any inefficiency that
>> should result in *that* much slowdown, and my own tests showed
>> virtually no difference between the two

>In a word, 'Predictor-1 compression'.  It's available on ijppp, and not
>with kernel-mode ppp and makes a significant difference.

Kernel-mode PPP has LZ/LZW compression (based on the /usr/bin/compress
code).  It works, but I'm not happy with it.  There is a subtle bug in
kernel-mode ppp that I've been unable to find, but it makes the
compression go to hell if it's active.

For example, if I sent a compressible text file uncompressed (over 11
bit LZW v42bus modems), I was getting about 5.6KB/sec (on 57.6K DTE
speed).  If I turned on software compression, and off on the modem
(28.8K), I was getting about than 2K/sec throughput.  The machine was
practically idle.  Somehow packets were getting excessively lost and
TCP was loosing the plot.  Most likely, I botched something with the
splxxx() locking.

I'd love to hear any suggestions..  I dont have the time to look at it
at the moment...

-Peter

>Nate