[Beowulf] substantial RX packet drops during Pallas over e1000 (Rocks 4.1)

[Dan Stromberg]

tcptrace is your friend.

One possibility is that your TCP windows are too small.

On Thu, 2006-05-18 at 15:03 -0700, David Kewley wrote:
> On Tuesday 16 May 2006 23:44, Jeff Johnson wrote:
> >    Packet drop example: (other nodes post similar numbers)
> >            RX packets:1843133 errors:0 dropped:1245 overruns:0 frame:0
> >            TX packets:1764828 errors:0 dropped:0 overruns:0 carrier:0
> 
> Question is: Where do these drops occur?
> 
> I've not looked into it in detail right now, but I will suggest to you that 
> this "dropped" statistic may represent packets that the kernel successfully 
> received and delivered to the application's socket receive buffer, but the 
> application did not remove these packets from the buffer before the buffer 
> overran.  At least, I've seen this behavior in other circumstances.
> 
> The "Recv-Q" and "Send-Q" columns in the output of netstat show you the 
> current size of data in the socket receive & send buffers.  I don't know if 
> there's a better way to keep an eagle eye on a particular socket's buffer 
> used sizes.
> 
> Adding lines something like the following to /etc/sysctl.conf may help you, 
> so long as the application is fundamentally able to keep up with the 
> average flow rate, and just needs a little help to get by brief periods of 
> high flow or longer packet pickup latency:
> 
> # Increase network write buffer max size from the default 128k to 512k
> net.core.rmem_default = 524287
> net.core.rmem_max = 1048575
> net.core.wmem_default = 524287
> net.core.wmem_max = 1048575
> 
> # Increase TCP write buffer max size from the default 128k to 512k
> net.ipv4.tcp_wmem = "4096 16384 524288"
> 
> Of course in your case we're concerned about the rmem values, not so much 
> about the wmem values.
> 
> David
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