Messages in this thread | | | Date | Thu, 04 Feb 1999 16:50:04 -0500 | From | Kris Karas <> | Subject | Re: Poor PPP performance in 2.2.x |
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Stephen Foskett wrote:
> There was some discussion of poor PPP performance in the 2.2.x > I tried disabling DMA... I tried the other settings with hdparm, too,
There are *two* separate problems, and one other possibliity I'll mention, here. One problem is interrupt latency that causes loss of serial bytes. The IDE driver is the usual culprit for this one; but you have evidently ruled that issue out, leaving some other piece of hardware as the likely culprit. Perhaps an ethernet interface? Try looking at /proc/net/interrupts to see if anything obvious appears. Any device in polled (PIO) mode is also suspect, e.g. a printer port.
Another possibility (particularly if the pppd on the remote end is invoked from a shell script) is that some other process is writing to the remote end's /dev/ttySx. I was perplexed by these a few years ago until I found some obnoxious mail notification programs were dumping output to the raw serial line; since pppd had put an entry into utmp linking the tty to my login account, the mail program found the line and wrote to it. But that doesn't look as though it's your problem either.
The second problem I alluded to is a bona-fide bug in the 2.1/2.2 tcp implementation. When the window size shrinks (as is the case when the modem buffers lots of serial data), tcp will periodically forget to send packets. I almost didn't believe it until I put a tcpdump on both ends of the connection and observed the packet loss first hand. But in this scenario, there are no errors reported by netstat/ifconfig, as the serial data itself has remained intact. I'm still waiting to hear something from the network gurus on this one.
Kris
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