I fired up Wireshark and took a look. I set up IPSEC to use
authentication only, so I can still see inside the packets. What I see,
on both server and client, is this:
When performance is poor, I see two fetch-data-64 packets from the server
followed by an ACK packet from the client. There is about a 4 ms delay
between the two fetch-data-64 packets. The sequence numbers are
consecutive and I see no sign of any retransmissions.
When performance is good, I see 8 or more fetch-data-64 packets in a row
followed by a bunch of ACK packets in return. The time between
fetch-data-64 packets is on the order of microseconds.
The 4 ms delay seems responsible, but I haven't figured out what might be
causing that.
Andrew pointed me to a link about IPSEC problems under RHEL 6. I do not
see the high ksoftirqd usage that the article mentions. I tried changing
/proc/sys/net/ipv4/xfrm4_gc_thresh, as they suggest, and got occasional
speedups but nothing repeatable.
thanks,
Steve Gaarder
System Administrator, Dept of Mathematics
Cornell University, Ithaca, NY, USA
gaar...@math.cornell.edu
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