Delta Temperature (DT) in Serial Presence Detect (SPD)3 is a throttling technique that is particularly beneficial in platforms that do not have a physical thermal sensor on the memory modules. At this point in time, not all memory modules have a physical thermal sensor on the DRAM modules monitoring their case temperatures; therefore, their temperature needed to be tracked by some alternative means. A Virtual Temperature Sensor (VTS) throttling mechanism [1] implemented in the chipset (first implemented in the Mobile Intel® 915 Chipset family), provides this alternative solution. VTS predicts memory case temperature based on the measured memory access type at each clock cycle and the speed and type of memory in the system. DT in SPD provides DRAM thermal data for each memory access type (Reads, Writes, Self Refresh, etc.) and includes the DRAM maximum case temperature limit data in the SPD on the memory module. These data are used by the VTS throttle mechanism (which analyzes memory traffic) to estimate the DRAM case temperature and to throttle memory based on the thermal prediction. DT in SPD better predicts the power and thermal of the memory module and hence achieves greater guardband reduction over the open-loop throttling methods followed in the previous-generation mobile platforms, and thus enhances performance in terms of bandwidth recovery. Our experiments clearly demonstrate a performance benefit of almost 15-30% (in terms of sustained bandwidth) by using DT in SPD.

Though DT in SPD has its benefits in platforms that don’t have a physical thermal sensor on the memory module, it is still an open-loop throttling technique since it does not actually know the operating temperature of the DIMM. Therefore it must always assume the laptop is operating under the maximum-allowed-room ambient temperature, which is typically considered to be 35°C for mobile environments. So if a notebook is actually running in an environment where the room ambient is only 20°C, then DT in SPD actually assumes it is running at 35°C and will initiate throttling ~15°C sooner than it needs to, thereby losing potential performance that could have been obtained had the system not initiated throttling so soon.

Thermal Sensor (TS) on Dual In-line Memory Module (DIMM)

TS on DIMM is the throttling technique that prevents memory from being over throttled and improves system performance. TS on DIMM integrates a physical thermal sensor onto the memory modules, providing real-time temperature information back to the chipset. The temperature feedback provides a closed-loop throttling mechanism that allows the system to adapt memory throttling to the actual environment the laptop is running in. So instead of just looking at memory traffic and estimating what the temperature of the DIMM might be based on estimated memory module power/thermals, now the chipset can wait to throttle until an actual thermal issue arises. This allows systems running low-power DIMMs, or systems with excellent cooling mechanisms, or systems running at cool room ambient temperatures, to run high-bandwidth applications and not have to throttle memory until a critical temperature is reached. So unlike DT in SPD, TS on DIMM does not have to assume the worst-case room ambient temperature. This allows TS on DIMM to greatly reduce the guardband required and thus improve system performance by allowing the system to run unconstrained for longer periods of time. Our experiments clearly demonstrate, due to the guardband reduction achieved using TS on DIMM, the total bandwidth recovered is as much as 30% of the theoretical maximum bandwidth (running a particular application on a specific system and memory configuration).

In the following sections, we discuss the need for system memory throttling and describe the concept, system implementation, and benefits of the two above-mentioned throttling techniques: DT in SPD and TS on DIMM.