Date: Sat, 29 Jun 1996 21:08:51 +1000 From: Bruce Evans <bde@zeta.org.au> To: jparnas@jparnas.cybercom.net, msmith@atrad.adelaide.edu.au Cc: Kevin_Swanson@BLaCKSMITH.com, chuckr@glue.umd.edu, freebsd-hardware@FreeBSD.org Subject: Re: muliport boards - building a PPP dialup server Message-ID: <199606291108.VAA02333@godzilla.zeta.org.au>
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>> These days, a multiboard should at least do 115200baud in my opinion. At >> least it can be competitive with modems. 8x115200baud is still only 1/8 >> MB/sec and 16x115200 is only 1/4 MByte/sec. Not exactly fast. >Fast has nothing to do with it. Interrupt rates do. >You should sit down and read some of the stuff that Bruce Evans has posted >on the subject over the years; most particularly his analysis of where the >actual load in handling serial ports comes from. Some key points : > - A 486 can service around 40,000 ISA interrupts per second, assuming > minimal interrupt processing time. > - Most of the CPU overheard in handling serial in/output is in the tty > layer. Erm, this shows that interrupt rates don't matter all that much (if the interrupt handler is efficient) except on machines with fairly fast CPUs (e.g. Pentiums) with fast i/o (e.g., 16.6MB/s for PIO mode 4) unless the UART's fifos are much too small (e.g. 8250s). Bruce
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