Greetings all,
I've been using real hipersockets for a couple years now. Recently a
significant performance problem with SLES 9 and large MTU sizes was brought
to my attention. I've been using MTU=32760. I set up a test to illustrate
the problem. I built a 256 MB file on one zLinux guest (running under z/VM
5.2 on a z9-109), with enough storage defined so the file could be
completely cached. I then FTP'd it to /dev/null on another server over a
hipersocket connection. Here's what I observed:
SLES8-to-SLES8, MTU=8184, ~75 MB/sec
SLES8-to-SLES8, MTU=32760, ~100 MB/sec (as high as 132 MB/sec)
SLES8-to-SLES9, MTU=32760, ~100 MB/sec
z/VM-to-SLES9, MTU=32760, ~100 MB/sec (from file on VDISK to /dev/null)
z/OS-to-SLES9, MTU=32760, ~25 MB/sec (from disk cache on z/OS)
SLES9-to-SLES9, MTU=8184, ~75 MB/sec
SLES9-to-SLES9, MTU=32760, ~400 KB/sec (not MegaBytes, KiloBytes!)
Has anyone else seen this?
I use 32760 on my hipersocket links so as to be consistent with the 32760
MTU size used on CTC links to other processors owned by my z/VM TCPIP
machine, which is used as the gateway between Linux guests on one machine
and z/OS on the other machines.
Best regards,
Mark Wheeler, 3M Company
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