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Solarflare Server Adapter User Guide
Solarflare Adapters on VMware
Issue 20 © Solarflare Communications 2017 284
Since the MTU should ideally be matched across all endpoints in the same LAN
(VLAN), and since the LAN switch infrastructure must be able to forward such
packets, the decision to deploy a larger than default MTU requires careful
consideration. It is recommended that experimentation with MTU be done in a
controlled test environment.
To change the MTU of the vSwitch, from the VMware Console OS enter:
#
esxcfg ‐ vswitch ‐‐ mtu <size> <vSwitch>
To verify the MTU settings, as well as obtaining a list of vSwitches installed on the
host, enter:
#
esxcfg ‐ vswitch ‐‐ list
The change in MTU size of the vSwitch will persist across reboots of the VMware ESX
host.
Interrupt Moderation (Interrupt Coalescing)
Interrupt moderation controls the number of interrupts generated by the adapter
by adjusting the extent to which receive packet processing events are coalesced.
Interrupt moderation may coalesce more than one packet ‐ reception or transmit ‐
completion event into a single interrupt.
By default, adaptive moderation is enabled. Adaptive moderation means that the
network driver software adapts the interrupt moderation setting according to the
traffic and workloads it sees.
Alternatively, you can set the moderation interval manually. You would normally
only do this if you are interested in reducing latency. To do this you must first disable
adaptive moderation with the following command, where
vmnicX
is the interface
name.
ethtool ‐ C <vmnicX> adaptive ‐ rx off
NOTE:
adaptive ‐ rx
may already have been disabled. Consult your VMware
documentation for details.
Interrupt moderation can be changed using ethtool, where vmnicX is the interface
name and interval is the moderation setting in microseconds ( μ s). Specifying 0 as
the interval parameter will turn interrupt moderation off:
ethtool –C <vmnicX> rx ‐ usecs ‐ irq <interval>
Verification of the moderation settings may be performed by running
ethtool –c
This parameter is critical for tuning adapter latency. Increasing the moderation value
will increase latency, but reduce CPU utilization and improve peak throughput, if the
CPU is fully utilized. Decreasing the moderation value or turning it off will decrease
latency at the expense of CPU utilization and peak throughput. However, for many
transaction request ‐ response type network applications, the benefit of reduced
latency to overall application performance can be considerable. Such benefits may
outweigh the cost of increased CPU utilization.