slow writes on nfs with bge devices

[Bruce Evans]

On Sun, 21 Jan 2007, Max Laier wrote:

> On Sunday 21 January 2007 13:25, Bruce Evans wrote:
>> On Sun, 21 Jan 2007, Max Laier wrote:
>>> On Sunday 21 January 2007 07:25, Bruce Evans wrote:
>>>> nfs writes much less well with bge NICs than with other NICs (sk,
>>>> fxp,
>>>
>>> Do you use hardware checksumming on the bge?  There is an XXX in
>>> bge_start_locked() that looks a bit suspicious to me.
>>
>> I use the default for that.  Wouldn't checksum problems show up as
>> errors somwhere?
>
> Did you look at the code in question?  It is concerned with fragmented
> packet chains (which NFS over UDP usually generated) and only commits to
> sending them, if there are enough descriptors available at once.  This
> can easily explain burstyness.
>
> Can you just try to disable the delayed checksums via "ifconfig -txcsum"?
> Should be an easy enough test.

I haven't looked closely at the bge checksum code, but tried turning off
checksums (various combinations) yesterday.  THis made no difference.

>> tcpdump shows a lot of intervals between nfs write requests of almost
>> exactly 10 mS (even with HZ = 1000, but more obvious with HZ = 100)
>> for the broken case, so the problem is apparently related to timeouts,
>
> See above, I really think that there is something about that if_start loop
> that might be causing this.

I have some local changes in this area (much larger ifq length and
watermark stuff to reduce tx interrupts).  I think they make no
difference, but will have to test an unchanged driver more carefully.
I assume that the if_start loop always calls the interface if_start
if there is something in the ifq and the interface is not marked as
active.

Bruce