PCIe 6.0’s thermal throttling plans could slam brakes on performance

Most enthusiasts know that a PC’s CPU will throttle down to prevent overheating. GPUs and even most PCIe 50. SSDs ship with huge heat sinks and blowers. But the latest casualty of thermal throttling might be the PCI Express bus itself — which is being programmed to reduce the link speed or even shut off some of its lanes.

That’s the eyebrow-raising scenario that a new driver from Intel suggests. Phoronix, which has been monitoring Intel’s development of a PCI Express driver for Linux, noted that the latest revision allows limiting the PCI Express link speed in case of thermal pressure. The key term is “bwctrl.”

That suggests that even a properly-cooled GPU or SSD might not be able to access as much as data as it normally would because the PCI Express link itself would slow down. Even worse, there is a possibility that the drivers could shut down some of the PCIe lanes themselves if necessary, to keep everything running cool. In other words, a PCI Express x16 connection could throttle down to a slower speed, then start turning off lanes — turning it into a x8 connection, possibly.

The notice on kernel.org notes that the latter won’t be possible until PCI Express 6.0, however — though that’s not too far away.

“This series adds PCIe bandwidth controller (bwctrl) and associated PCIe cooling driver to the thermal core side for limiting PCIe Link Speed due to thermal reasons,” Intel engineer Ilpo Järvinen wrote. “PCIe bandwidth controller is a PCI express bus port service driver. A cooling device is created for each port the service driver finds if they support changing speeds. This series only adds support for controlling PCIe Link Speed. Controlling PCIe Link Width might also be useful but AFAIK, there is no mechanism for that until PCIe 6.0 (L0p) so Link Width throttling is not added by this series.”

The PCI Express 6.0 specification was announced in 2022, though hardware has yet to arrive. (That should happen soon, however, as the PCI-SIG originally projected March 2024 for devices to begin supporting that specification.) Right now, Intel appears to be eyeing its cooling solution with servers in mind.

But PCIe 5 SSDs exist, and they already need dedicated coolers. It’s not too hard to believe that PCIe 6 will need even more aggressive cooling, and PCI Express bandwidth throttling and link-width control may be an unfortunate side effect in extreme scenarios.