From James Melin's 12/14/2006 post:
Mark, I saw something similar when we first went to SLES-9 (64 bit). We had
MTU sizes being negotiated down to an idiotic packet size of 1492.
Whether or not this has to do with how our OSA is configured I never did learn.
Search for stuff with hipersockets and MTU in the archives for the
particulars.
We reported it to Novell. They eventually managed to test on SLES 9 under
z/Series and eventually issued a patch. They sent us this file:
iputils-ss021109-147.2.PTF.184411.0.s390.rpm
I don't know whether or not this file is GA but it touched MANY IP related
items, indicating that the problem was pervasive. Once we applied it our
MTU negotiation problem went away. It may or may not have bearing on your
situation, but I offer the information to any and all that might find it
relevant.
-J
barton wrote:
I've seen this a lot. You need to find the real bottleneck. Are
you equiped?
Mark Wheeler wrote:
More info:
FTP GET's of this cached 256MB file to /dev/null have run as fast as 205
MB/sec w/ MTU=32760 over hipersocket interfaces, and 168 MB over virtual
CTCs. FTP PUT's are a different matter: hipersocket or CTC doesn't
make a
difference. The higher MTU size, the slower the transfer rate (from
90-100
MB/sec w/ MTU=16376 down to 10-15 MB/sec for MTU=32084. At MTU=32085,
performance drops off a cliff, down to 400 KB/sec.
So, one problem appears to be related to FTP PUT performance with
large MTU
sizes.
In the process of investigating this alleged "hipersocket performance
problem" (as reported by our Linux support person up four chains of
management), I discovered an interesting issue with scp. It shows very
limitted sensitivity for interface type, MTU size, or direction of
movement), however I can't get it to run faster than 8 MB/sec (25 times
slower than FTP GET). I'm using scp between two virtual Linux machines
running on a 1-IFL system, so thought initially it may have been CPU
contention (no hardware crypto on this box), yet CPU util was less
than 5%
on each machine. What I did see was the sending side doing a lot of disk
I/O, something I don't see with FTP. Is it possible that scp reads from
disk, bypassing the cache?
Linux version 2.6.5-7.276-s390x
Best regards,
Mark
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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