svn commit: r273112 - head/sys/dev/ixgbe

[Alexander V. Chernikov]

> On 07 Nov 2014, at 03:15, Luigi Rizzo <rizzo at iet.unipi.it> wrote:
> 
> On Thu, Nov 06, 2014 at 03:05:30PM +0400, Alexander V. Chernikov wrote:
>> On 15.10.2014 05:22, Adrian Chadd wrote:
>>> Author: adrian
>>> Date: Wed Oct 15 01:22:56 2014
>>> New Revision: 273112
>>> URL: https://svnweb.freebsd.org/changeset/base/273112
>>> 
>>> Log:
>>>   Set the DROP_EN bit before the RX queue is brought up and active.
>>> 
>>>   He noticed issues setting this bit in SRRCTL after the queue was up,
>>>   so doing it from the sysctl handler isn't enough and may not actually
>>>   work correctly.
>> While investigating/measuring ixgbe performance w/ and without flow control
>> I noticed that disabling fc entirely (e.g. disabling 512K skid buffer 
>> and turning drop_en on)
>> not only makes things a bit worse, but also makes NIC stop accounting 
>> tail-drops as errors
>> at all.
>> It is a bit unhandy with HW counters, since you, for example, see 2M 
>> packets being received
>> in netstat, but in reality system receives only 1M (And there is no easy 
>> way to determine exact
>> count).
> 
> i'd like to point out that the default does not change (flow control
> is still enabled by default) and the previous handling of DROP_EN
The default did change: now drop_en is turned on by default due to a small mistake (see my comment to commit).
> was ineffective.
> 
> One may argue that FC and DROP_EN should be set independently,
> and i am all for it. In fact that would be the right thing to do
> in terms of POLA.
> 
> This said, i do not understand what kind of performance degradation
> you experienced, can you be more specific ?
Well, I haven’t tested it in great details, but for my traditional shooting (IPv4 200 dsts to single rte) I observed some notable change,
like 9.2->8.6 MPPS or similar. I’m not insisting that something should be done about it, since there is no silver bullet here.
My point was that we’ve lost an ability to count ingress errors in stock HEAD.

> 
> cheers
> luigi