[Click] RX Overruns with eepro100cd

[Chip Coldwell]

On Fri, 1 Aug 2003, Barry Gowan wrote:
> 
> So _dev->rx_refill() blocks while a memory access on
> the card is done to refill the ring buffer in the
> driver.

I'm not sure if I could call this "blocking", since there's no wait
queue or sleeping process involved, although I suppose in principle
the kclick kernel thread could get pre-empted during this operation if
there's a timer interrupt.  I think the kclick priority is high enough
that it would always be rescheduled, though.

> So is there nothing I can change to improve this
> situation? If I had a faster PCI bus would I get a
> better result? Or is it just down to the eepro100 NIC
> being too busy to serve the memory fast enough?

Click will sometimes explicitly call the OS scheduler.  You can find
the logic in RouterThread::driver, where it will alternate between
calls to "run_tasks" (run the Click tasks) and "run_os" (call the OS
scheduler).  We rely on run_tasks to call PollDevice::run_task which
in turn calls _dev->rx_refill, so if the Click scheduler decides to
call the OS scheduler instead that introduces an unpredictable amount
of delay between successive calls to _dev->rx_refill and can lead to
overruns.  If you have an SMP computer with Click configured to use
just one CPU, then you can use the "--enable-greedy" configure option
to eliminate the calls to the OS scheduler from within the Click
scheduler.  Basically this gives one CPU to the OS and one to Click,
and my experience has been that Click performance is greatly improved.

> When I replace the eepro100 with e1000 NICs, the same
> test runs fine. I guess this is because whatever DMA
> that _dev->rx_refill() triggers runs faster on the
> e1000 hardware than the eepro100. Have I got this
> analysis correct?

In the case of the E1000, one register write per sk_buff is required
(those are writes across the PCI bus).  I haven't looked at the
eepro100 driver, but it might be using more bus transfers, which would
impact performance.

Chip

-- 

Charles M. "Chip" Coldwell
"Turn on, log in, tune out."