Issue 11
© Solarflare Communications 2014
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Solarflare Server Adapter
User Guide
receive the same amount of data. Solarflare adapters supports maximum frame sizes up to 9216
bytes (this does not include CRC).
Since the maximum frame size should ideally be matched across all endpoints in the same LAN
(VLAN) and the LAN switch infrastructure must be able to forward such packets, the decision to
deploy a larger than default maximum frame size requires careful consideration. It is recommended
that experimentation with maximum frame size be done in an application test environment.
The maximum frame size is changed by changing the Max Frame Size setting in the Network
Adapter’s Advanced Properties Page.
Interrupt Moderation (Interrupt Coalescing)
Interrupt moderation reduces the number of interrupts generated by the adapter by coalescing
multiple received packet indications and/or transmit completion events together into a single
interrupt. The amount of time the adapter waits after the first event until the interrupt is generated
is the interrupt moderation interval.
Solarflare adapters, by default, use an adaptive algorithm where the interrupt moderation delay is
automatically adjusted between zero (no interrupt moderation) and 60 microseconds. The adaptive
algorithm detects latency sensitive traffic patterns and adjusts the interrupt moderation interval
accordingly. The adaptive algorithm can be disabled to reduce jitter and the moderation interval set
higher/lower as required to suit conditions.
For lowest latency, interrupt moderation should be disabled. This will increase the number of
interrupts generated by the network adapter and as such increase CPU utilization.
Interrupt moderation settings are
critical for tuning adapter latency
. Increasing the moderation
time may increase latency, but reduce CPU utilization and improve peak throughput, if the CPU is
fully utilized. Decreasing the moderation time 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 typically outweigh the cost of increased CPU utilization.
Interrupt moderation can be disabled by setting the Interrupt Moderation setting to disabled in the
Network Adapter’s Advanced Properties Page. The interrupt moderation time value can also be
configured from the Network Adapter’s Advanced Properties Page.
Interrupt Moderation Interval
The interrupt moderation interval is measured in microseconds. When the interval expires the
adapter will generate a single interrupt for all packets received since the last interrupt and/or for all
transmit complete events since the last interrupt.
Increasing the interrupt moderation interval will:
NOTE:
The maximum frame size setting should include the Ethernet frame header. The Solarflare
drivers support 802.1p. This allows Solarflare adapters on Windows to optionally transmit packets
with 802.1p tags for QOS applications. It requires an Ethernet frame header size of 18bytes
(6bytes source MAC address, 6bytes destination MAC. 2bytes ethertype and 4bytes priority tag.
The default maximum frame size is therefore: 1518 bytes.