Intel DMA Coalescing White Paper

• Set one port as Receive with smartbits
= 1514 byte continuous UDP packet
stream from another port.
• Results & Observations
• Throttling interrupts by itself improves
power efficiency.
• Adding DMA coalescing creates further
power savings. Figure 4 shows how
moderating interrupts improves power
efficiency and the addition of DMA
coalescing further increases power
savings.
• Peak benefit reached at expected
throughput of ~250 Mbps (25%),
• Beyond optimal throughput, power
savings begins to decrease. Figure 4
shows the power savings of a single
port using interrupt moderation and
DMA coalescing within the context of
network usage.
• DMA moderation benefits increase as
more time is allowed for coalescing,
for example 250 uS to 5 mS. However,
as additional time for coalescing is
enabled, response-time latency (if the
network data is not sufficient to exceed
the device water mark) increases
proportionally.
• Asynchronous activity between two
discrete controllers (2x dual-port vs
1x quad-port) will interfere with CPU
lower power state entry and duration,
reducing DMA coalescing power
effectiveness.
Intel® Ethernet I350 Controller
• Integrated Quad Port Silicon
• Intel has achieved DMA Coalescing in an
integrated quad port part today!
• Intel synchronizes DMA activity across
all four ports of our quad port controllers
beginning with the I350
DMA Coalescing Across Multiple Intel
Quad Port Adapters
• Through software emulation, Intel is able
to synchronize DMA Coalescing between
two Intel adapters
When the OS selects entry into ACPI C3,
the BIOS will map this request to the
internal CPU C6 state
2. Enable Package C3 and Package C6
This enables the CPU to select, synchro-
nize, and activate a low power mode over
multiple CPU cores simultaneously.
3. Enable Enhanced Intel Speedstep Tech-
nology (EIST).
Enhanced Intel SpeedStep Technology
enables the system to dynamically adjust
processor voltage and core frequency.
This can result in decreased average
power consumption and decreased
average heat production.
4. Enable ASPM L1 if possible for
additional PCIe power savings.
Software Operating System Tuning
When using Windows* Server 2008 R2:
1. Disable core parking if needed.
2. Install all chipset-specic and device-
specic device drivers (such as the
Intel® Chipset INF updater, as well as
vendor-specic graphics drivers).
Contact your local Intel Field
representative to obtain the “SelfTest”
tool from http://www.intel.com/cd/
edesign/library/asmo-na/eng/434688.htm
. The tool veries the platform BIOS/OS
Figure 4
• Typical platform power savings of 15 W
to 20 W per server with DMA Coalescing
enabled on a single four port LAN device
• Additional testing results and details will
be forthcoming in future revisions of this
document
Additional Configuration
Information
The following platform-level
congurations and settings dramatically
improve the power efciency of a system
using Intel PMT.
Platform Considerations
Overall, minimize the use of USB* devices.
The USB bus is a polled bus; transactions
are initiated by the host and not the
USB device. Because of this, USB devices
contribute more interrupts to the system
and make it difcult to control Power
Managment. USB 2.0 does support a
“suspend” low-power state; but the state’s
entry/exit latency make it difcult to
use effectively. Best results occur when
network applications scale across multiple
CPU cores as evenly as possible. Enable
Receive Side Scaling (RSS) to afnitize
interrupts to the CPU cores.
BIOS Tuning
The following settings are typically
congured in the BIOS setup screens:
1. Enable C1E, disable C3-report, and
enable C6-report
4
Intel® I350 Ethernet Controller & DMA Coalescing