Issue 11
© Solarflare Communications 2014
259
Solarflare Server Adapter
User Guide
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
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.
NOTE:
adaptive-rx
may already have been disabled. Consult your VMware documentation for
details.
NOTE:
The interrupt moderation time dictates the minimum gap between two consecutive
interrupts. It does not mandate a delay on the triggering of an interrupt on the reception of every
packet. For example, an interrupt moderation setting of 30
μ
s will not delay the reception of the
first packet received, but the interrupt for any following packets will be delayed until 30
μ
s after the
reception of that first packet.