Subject: Transmit interrupts, fxp driver for Intel 82557/8/9 Ethernet
To: None <firstname.lastname@example.org>
From: Hal Murray <email@example.com>
Date: 05/09/2000 00:43:13
The fxp driver doesn't use transmit interrupts. Under normal traffic
patterns this works fine. The work gets done on the next receive
interrupt, saving the CPU cycles that would have been burned by the
overhead on each transmit interrupt.
But it doesn't work very well if you run crazy test programs that
only send UDP packets in one direction. The watchdog timer eventually
catches things, but that takes a long long time and it puts a lot
of "fxp0 timeout" messages on the screen.
Netperf includes a test like that. It's a simple way to check for
the classic live-lock troubles where the receiver spends all its
CPU cycles processing a packet only to discover that the input socket
queue is full so it drops the packet.
Has anybody else encountered this quirk? Is this enough of a bug
that I should send a PR?
I found the place in the code where it sets up the transmit command.
(That's not hard, the symbol is only used once.) When I or-ed in
the interrupt bit, things worked as expected. (I forget to measure
how much more CPU that used.)
I tried turning on the interrupt bit when the transmit queue was
3/4 full but either I botched the code or that wasn't a good enough
I think I've also discovered a variation of this glitch. If the
TCP window is big enough, throughput drops way off. The break happens
between 190K and 195K. That's slightly over 128 packets which is
the size of the fxp transmit queue.
On a pair of 400 MHz Celerons, throughput drops from 90 megabits
with the window under 190K bytes to under 20 when the window is over