jamal wrote:
On Tue, 2006-04-07 at 13:11 -0400, jamal wrote:
I have a device connected to a e1000 that was erroneously advertising
both tx/rx flow control but wasnt properly reacting to it.
The default setup on the e1000 has rx flow control turned on.
I was sending at wire rate gige from the device - which is about
1.48Mpps. The e1000 was in turn sending me flow control packets
as per default/expected behavior. Unfortunately, it was sending
a very large amount of packets. At one point i was seeing upto
1Mpps and on average, the flow control packets were consuming
60-70% of the bandwidth. Even when i fixed this behavior to act
properly, allowing flow control on consumed up to 15% of the bandwidth.
Clearly, this is a bad thing. Yes, the device in the first instance was
at fault. But i have argued in the past that NAPI does just fine without
flow control being turned on, so even chewing 5% of bandwidth on flow
control is a bad thing..
As a compromise, can we declare flow control as an advanced feature
and turn it off by default? People who feel it is valuable and know
what they are doing can turn it off.
I meant turn it on.
BTW, As an addendum this default behavior changed around 2.6.16 it
seems.
Flow Control is using the EEPROM provided value, the module driver itself does
not choose a default:
e1000_param.c:
/* User Specified Flow Control Override
*
* Valid Range: 0-3
* - 0 - No Flow Control
* - 1 - Rx only, respond to PAUSE frames but do not generate them
* - 2 - Tx only, generate PAUSE frames but ignore them on receive
* - 3 - Full Flow Control Support
*
* Default Value: Read flow control settings from the EEPROM
*/
Turning flow control off usually (i.e. almost always) causes (significantly)
_degraded_ performance. We should really leave it the way it is (as per eeprom
setting), and this is best for most if not all people. The card itself has
this value programmed, which makes it possible for the user to turn on/off
flowcontrol per card consistently, which makes much more sense to me. Also
considering e1000 hardware varies significantly.
Moreover, most support calls here where people complain about degraded
performance are about users who turned flow control _off_. :)
Cheers,
Auke
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