On 10/12/06, Heiko wrote:
>Short description: The Mac is a OS X.4 client with 100MBit network
>connection (172.27.2.127), and the Sun is Solaris 10 AFP-Server
>(Ethershare) with a 1000MBit network connection (172.27.1.40). The
>problem only occurs while reading data from the Sun.
Solaris uses Mentat/TCP while Mac OS X uses a variation of the BSD
stack. Both are fairly tunable but may have different assumptions
about high bandwidth low latency pipes.
>The Mac acknoledges data packets from the Sun very lazily, meaning
>the Sun sends up to 6 packets, the Mac ack's just one. This way the
>retransmission queue of the sun fills up with unacknoledged packets,
>forcing the Sun to throttle down.
>Some packets later, the Sun only sends new packets if prior packets
>are acknoledged.
One obvious suggestion would be to turn off delayed ACK on the Mac OS X box.
sudo sysctl -w net.inet.tcp.delayed_ack=0
You can do this using UNIX sysctl or my own IPNetTunerX
(www.sustworks.com) which will give you better visibility into the
parameters available and the effect of tuning.
I noticed just now on 10.4.8 that the value for delayed Ack is "3" which
surprises me. Previously I only noticed it could be "on" (1) or
"off" (0). This may have some bearing on why Mac OS X is ACKing so lazily.
>Now the Mac sends acks for more then one packet at a time (good
>thing), but delays the send about 500microseconds...this gets worse,
>until the delay is about 200ms (milliseconds!). As you can imagine,
>the throughput is near zero.
This certainly looks like a delayed_ack issue. I've just never seen the
delay become quite so large. Are your two machines directly attached to
the same LAN, or is there a router between them? I ask because TCPs
slow start behavior is different for a host on a non-local network.
>Has anyone heard of such a problem before?
There are some known tweaks to speed up Samba file copying speeds.
Hopefully the above pointers will get you started.
Kind Regards,
- Peter
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