Original 45nm Intel® Core™ Microarchitecture

Power Improvements on 2008 Desktop Platforms

USB IMPACT ON PLATFORM POWER AND C-STATE RESIDENCY

An additional power-savings opportunity can be had by enabling the USB selective suspend feature in the OS [7] . Selective suspend is a low-power mode defined in the USB 2.0 specification [8] . that allows the USB hub driver to turn off USB ports when they are in idle. (Figure 9) illustrates these power savings. Observe that the Yorkfield processor shows a 76-percent power savings in these conditions (an additional 16 percent over what was shown in (Figure 7) ). This, along with the Intel® Q45 Express Chipset's 34-percent improvement and the system memory's 73-percent power savings, help to achieve a 22-percent power improvement at the AC platform level.

This improvement in the component power is also reflected in the C-state residency data with USB selective suspend enabled, as shown in (Figure 10) . C4 state residency now is just over 98 percent compared to 88 percent achieved before without USB selective suspend enabled, and this translates into the power savings seen in (Figure 9).

Figure 9: Platform and component power savings with USB selective suspend enabled

Figure 10: Effect of USB selective suspend on C-state residency

Enabling this feature in the OS also helps to increase the active power savings as illustrated in (Figure 11) . For Sysmark 2007 we see a 7-percent improvement in the AC platform power on average as compared to the 5-percent improvement seen earlier (refer to (Figure 8) ).

Figure 11: Sysmark power savings with USB selective suspend enabled

In addition to the USB hub device behavior, USB devices such as the keyboard and mouse have an adverse impact on the platform power management because of the polling architecture of USB devices [9] . USB devices cannot initiate a transfer or transfer data without being polled by the host first. Because of this, the processor has to constantly come out of the C4 state just to poll the USB devices and check whether they have any data to transfer, thereby incurring a power penalty, especially in idle conditions. So while a 76-percent power savings was achieved by using a PS2 keyboard mouse and by enabling USB selective suspend in the OS, only a 69-percent power savings for the processor is observed when a USB keyboard mouse are connected to the platform with USB selective suspend enabled, and similar behavior is observed for the other components as well. As a result, only a 16-percent power savings is seen for the AC platform power. (Figure 12) illustrates the power savings seen when the PS2 and USB keyboard and mouse are connected to the 2008 platform.

Figure 12: Power savings with PS2 and USB Keyboard/Mouse—USB selective suspend enabled

This change in power savings can also be seen from the C-state residency numbers as illustrated in Figure 13 . There is clearly a change in the C-state residency when a USB keyboard and mouse are added to the system—88-percent C4 residency and 11-percent C2 residency compared to the 98.4-percent C4 residency and 0.8-percent C2 residency seen with a PS2 keyboard mouse. This large shift in the C2 state residency explains the reduced power savings seen at the platform and component level.

Figure 13: C-state residency with USB Keyboard/Mouse—USB selective suspend enabled

Figures 6 and 9 demonstrate how the 2008 platform is improved with respect to platform power management and along with (Figure 12) establish that we get the most power savings when we use the 2008 platform with the USB selective suspend feature enabled in the OS and with a PS2 keyboard and mouse.

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