serious networking (em) performance (ggate and NFS) problem

Andre Oppermann andre at
Thu Nov 25 10:48:21 PST 2004

Robert Watson wrote:
> On Sun, 21 Nov 2004, Sean McNeil wrote:
>>I have to disagree.  Packet loss is likely according to some of my
>>tests.  With the re driver, no change except placing a 100BT setup with
>>no packet loss to a gigE setup (both linksys switches) will cause
>>serious packet loss at 20Mbps data rates.  I have discovered the only
>>way to get good performance with no packet loss was to
>>1) Remove interrupt moderation
>>2) defrag each mbuf that comes in to the driver.
> Sounds like you're bumping into a queue limit that is made worse by
> interrupting less frequently, resulting in bursts of packets that are
> relatively large, rather than a trickle of packets at a higher rate.
> Perhaps a limit on the number of outstanding descriptors in the driver or
> hardware and/or a limit in the netisr/ifqueue queue depth.  You might try
> changing the default IFQ_MAXLEN from 50 to 128 to increase the size of the
> ifnet and netisr queues.  You could also try setting net.isr.enable=1 to
> enable direct dispatch, which in the in-bound direction would reduce the
> number of context switches and queueing.  It sounds like the device driver
> has a limit of 256 receive and transmit descriptors, which one supposes is
> probably derived from the hardware limit, but I have no documentation on
> hand so can't confirm that.
> It would be interesting on the send and receive sides to inspect the
> counters for drops at various points in the network stack; i.e., are we
> dropping packets at the ifq handoff because we're overfilling the
> descriptors in the driver, are packets dropped on the inbound path going
> into the netisr due to over-filling before the netisr is scheduled, etc. 
> And, it's probably interesting to look at stats on filling the socket
> buffers for the same reason: if bursts of packets come up the stack, the
> socket buffers could well be being over-filled before the user thread can
> run.

I think it's the tcp_output() path that overflows the transmit side of
the card.  I take that from the better numbers when he defrags the packets.
Once I catch up with my mails I start to put up the code I wrote over the
last two weeks. :-)  You can call me Mr. TCP now. ;-)


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