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Solarflare Server Adapter User Guide
Solarflare Adapters on FreeBSD
Issue 20 © Solarflare Communications 2017 307
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.
Interrupt moderation settings are critical for tuning adapter latency :
• 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.
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. It is recommended
that:
• Interrupt moderation is disabled for applications that require best latency and
jitter performance, such as market data handling.
• Interrupt moderation is enabled for high throughput single (or few) connection
TCP streaming applications, such as iSCSI.