Network performance degradation with IBM s390x systems when TSO turned on

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Environment

SUSE Linux Enterprise Server 12
SUSE Linux Enterprise Server 12 Service Pack 1 (SLES 12 SP1)

Situation

For certain workloads the network performance of IBM s390x systems is degraded when TSO is turned on. The issue is triggered by a combination of the lower scheduler frequency on the s390x architecture and fast network devices and connection. In systems exhibiting this issue the TCP connection throughput is very low (as much as 30 times lower).

Resolution

The best workaround known at this time is to increase the value of net.ipv4.tcp_min_tso_segs to 32.

On a running system, one can use either

echo 32 >/proc/sys/net/ipv4/tcp_min_tso_segs

sysctl -w net.ipv4.tcp_min_tso_segs=32

To make the change permanent, either add line "net.ipv4.tcp_min_tso_segs = 32" to /etc/sysctl.conf, or the preferred solution is to create a new file with ".conf" extension containing "net.ipv4.tcp_min_tso_segs = 32" and put it in /etc/sysctl.d.

Disclaimer

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