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Solarflare Server Adapter User Guide
Solarflare Adapters on Linux
Issue 20 © Solarflare Communications 2017 105
Interrupt Moderation (Interrupt Coalescing)
Interrupt moderation reduces the number of interrupts generated by the adapter by
coalescing multiple received packet events and/or transmit completion events
together into a single interrupt.
The interrupt moderation interval sets the minimum time (in microseconds)
between two consecutive interrupts. Coalescing occurs only during this interval:
• When the driver generates an interrupt, it starts timing the moderation
interval.
• Any events that occur before the moderation interval expires are coalesced
together into a single interrupt, that is raised only when the interval expires.
A new moderation interval then starts, during which no interrupt is raised.
• An event that occurs after the moderation interval has expired gets its own
dedicated interrupt, that is raised immediately.
A new moderation interval then starts, during which no interrupt is raised.
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.
Interrupt moderation settings are critical for tuning adapter latency :
• Disabling the adaptive algorithm will:
‐ reduce jitter
‐ allow setting the moderation interval as required to suit conditions.
• Increasing the interrupt moderation interval will:
‐ generate less interrupts
‐ reduce CPU utilization (because there are less interrupts to process)
‐ increase latency
‐ improve peak throughput.
• Decreasing the interrupt moderation interval will:
‐ generate more interrupts
‐ increase CPU utilization (because there are more interrupts to process)
‐ decrease latency
‐ reduce peak throughput.
• Turning off interrupt moderation will:
‐ generate the most interrupts
‐ give the highest CPU utilization
‐ give the lowest latency
‐ give the biggest reduction in peak throughput.