Solarflare SFA6902F, User Manual, Page: 290 / 375

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Solarflare Server Adapter
User Guide
Tuning settings
Adapter MTU (Maximum Transmission Unit)
The default MTU of 1500 bytes ensures that the adapter is compatible with legacy 10/100Mbps
Ethernet endpoints. However if a larger MTU is used, adapter throughput and CPU utilization can be
improved. CPU utilization is improved because it takes fewer packets to send and receive the same
amount of data. Solarflare adapters support frame sizes up to 9000 bytes (this does not include the
Ethernet preamble or frame-CRC).
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.
The MTU can be changed dynamically using
ifconfig
if the maximum MTU size has been set in the
sxfge_conf
file (see Configuring Jumbo Frames on page 269 ), where
sfxge<X>
is the interface
name and size is the MTU size in bytes:
$ ifconfig sfxge<X> mtu <size>
Verification of the MTU setting may be performed by running
$ ifconfig sfxge<X>
with no
options and checking the MTU value associated with the interface. If you want to have an MTU
configure when the interface is brought up, add
mtu
to the single line of configuration data in:
/etc/hostname.sfxge<X>
For example:
[<IP address>] mtu <size>
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.
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: 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.