POWER9/Overclocking

Overclocking

Legal Notice

Raptor Engineering, LLC, Raptor Computing Systems, LLC, IBM, and affiliated entities disclaim all liability resulting from the use or misuse of this information. You accept all risk associated with modification of clock and/or power data, including, without restriction, hardware damage, data corruption, and/or loss of business resulting from the use of this information.

CPU and/or mainboard damage incurred as a result of attempted overclocking may void any and all warranty(s) for the damaged component(s).

Overview

Overclocking a POWER9 processor is a somewhat complex task as the existing WoF clock management software in the OCC remains active, and is already attempting to push the processor to its VPD-specified clock limits during periods of light load. As such, there are two primary ways to overclock a POWER9 processor: adjusting the WoF tables and processor VPD (WoFerClock) or the application of a simple offset to all voltages and frequencies (BiasClock). Setting up WoFerClock is a more involved process but generally yields better results; BiasClock is comparatively simple but only works for small overclocks due to WoF Phase 2 frequency boosting.

Known Constraints

• Sforza modules ship with a nest frequency of 1866MHz (VPD bucket 2).
• Core clocks cannot exceed twice the nest frequency.
• WoFerClock copies modified Bucket 2 data to Bucket 5.
• WoF works by clipping the Pstate frequencies calculated from the VPD and bias data. It does not boost over the calculated data regardless of table contents.

General Hints

• 4.2GHz should be readily achievable on most Sforza parts. Higher clocks have not yet been stably achieved. It is believed the 4 and 8 core parts are the best candidates for a "speed run", i.e. an attempt to reach 5GHz.
• Exceeding 220W-230W on the Sforza parts is not recommended. There is some margin remaining on Rev 1.01 Talos II mainboards up to around the 250W range, but adequate cooling for the regulators MUST be provided to avoid triggering a safety shutdown.
• Naively altering the WoF tables to simply increase frequency clips will also increase voltage and, therefore, power consumed by the processor. It is very easy to overheat a processor by only altering the WoF tables, and using the provided overclock methods in lieu of direct WoF table alteration is strongly recommended.

BiasClock

Resources

• BiasClock README

WoFerClock

Using WoFerClock is a two step process. First, the module VPD needs to be updated -- this is the CPU Vital Product Data that stores operating point information. Second, the new nest frequency and WoF tables must be loaded into the PNOR. If the overclock is unstable, the original PNOR can be reflashed, restoring operation on Bucket 2 and known good frequency / voltage data.

The main WoFerClock program is preconfigured to enable a 2400MHz PowerBus (nest) frequency and a 4.2GHz Ultra Turbo frequency; these values are shared with the "raptor-aggressive" machine XML. Both the WoFerClock values and the WoF tables must be configured with the same core frequency and nest frequency, otherwise WoF will not engage. The WoF tables in the raptor-aggressive branch are generated with the generate_aggressive_tables script.

WoFerClock is less useful for the 4 and 8 core CPUs since they are preprogrammed to boost to the maximum Ultra Turbo frequency regardless of number of loaded cores.

Resources

• WoFerClock README