Enabling Priority Core Turbo (PCT) for GPU Performance

Overview

Intel® Priority Core Turbo (PCT) is part of Intel® Speed Select Technology – Turbo Frequency (SST-TF). It allows a subset of CPU cores to operate at higher turbo frequencies, while remaining cores run closer to base frequency.

This is particularly effective for GPU-accelerated AI inference, where a small number of CPU threads handle latency-critical, mostly serial tasks such as tokenization, scheduling, and feeding GPUs. Running these threads on High-Priority (HP) cores improves GPU utilization, Time-to-first-token (TTFT), and tail latency.

This guide provides a Docker-based workflow and supporting scripts to check PCT support, configure CLOS assignments, and validate the selected high-priority CPU list. The PCT setup and verification steps run from the container, so users do not need to install intel-speed-select or other extra host-side dependencies beyond Docker.

Validated platforms:

How PCT Works

PCT Details PCT relies on **two Intel Speed Select features**: - **SST-TF (Turbo Frequency)** Defines the high-priority turbo buckets and the number of physical cores that can use each bucket. - **SST-CP (Core Power / CLOS)** Assigns CPUs to **Classes of Service (CLOS)**. CPUs assigned to **CLOS0** are treated as **High-Priority** by PCT. > **Important:** PCT is only effective when CPUs are explicitly assigned to **CLOS0** and Core Power / CLOS is enabled. ### PCT Bucket-Count Interpretation intel-speed-select tool mentioned below is installed inside the docker image in the [environment build seciton](#1-build-the-environment) `intel-speed-select turbo-freq info -l ` may print the same `bucket-0`, `bucket-1`, and `bucket-2` SST-TF table under multiple `powerdomain-*` anchors. A powerdomain anchor is the representative CPU id where a packages's internal power domain starts. For PCT **capacity**, this flow counts `bucket-0` **once per package/socket**: ```text bucket-0 high-priority-cores-count:8 @ 4600 MHz => 8 PCT physical cores per package/socket ``` On a two-socket Intel® Xeon® 6776P system with Hyper-Threading enabled: ```text 2 packages × 8 physical PCT cores/package = 16 physical PCT cores total 16 physical PCT cores × 2 threads/core = 32 logical PCT CPUs total ``` ### Capacity Versus Placement There are two different concepts: | Concept | Correct model | | --- | --- | | **PCT capacity** | Count `bucket-0` once per package/socket | | **HP CPU placement** | Dispatch the package-level PCT core budget across the package's PCT reporting powerdomain anchors | The excerpts below are from execution of [check_pct_status.sh](/optimization-zone/hardware/priority_core_turbo/check_pct_status.sh) on an Intel® Xeon® 6776P system with 2 sockets and 64 cores per socket. Subsequent sections explain the outputs and how to interpret them. ```text PCT_CORES_PER_PACKAGE=8 PCT_ACTIVE_PACKAGES=2 PCT_TOTAL_PHYSICAL_CORES=16 THREADS_PER_CORE=2 PCT_TOTAL_LOGICAL_CPUS=32 ``` But the `turbo-freq` output shows two reporting anchors per package: ```text package 0: anchor cpu0, anchor cpu32 package 1: anchor cpu64, anchor cpu96 ``` Therefore, the set script dispatches the **8 physical PCT cores per package** across the package's two reporting anchors: ```text package 0: 4 physical cores from cpu0 + 4 physical cores from cpu32 package 1: 4 physical cores from cpu64 + 4 physical cores from cpu96 ``` With Hyper-Threading included, this becomes: ```text 0-3,32-35,64-67,96-99,128-131,160-163,192-195,224-227 ``` </details> ## 1. Build The Environment
Build Details Export the kernel build variables first: ```bash source ./set_kernel_env.sh ``` Build the Docker image with required tools: ```bash docker compose --progress=plain build --no-cache ``` Verify `intel-speed-select` exists inside the image: ```bash docker compose run --rm intel-speed-select-shell 'which intel-speed-select && intel-speed-select --help | head' ```
## 2. Check PCT Status
This step verifies: - Hardware support for Intel® Speed Select features - SST-TF/PCT bucket-0 capacity - Correct package/socket-based PCT capacity counting - Core Power and CLOS enablement - Current CPU-to-CLOS mapping - Whether the current `TARGET_CLOS` CPU count matches the expected PCT logical CPU budget
Export the kernel build variables first: ```bash source ./set_kernel_env.sh ``` Run: ```bash docker compose --progress=plain --profile check up --abort-on-container-exit ``` Example results when PCT and CLOS are enabled successfully:
Example results ```bash ------------------------------------------------------------ CPU and Intel Speed Select Capability ------------------------------------------------------------ Intel(R) SST-PP (feature perf-profile) is supported Intel(R) SST-TF (feature turbo-freq) is supported Intel(R) SST-BF (feature base-freq) is not supported Intel(R) SST-CP (feature core-power) is supported Intel(R) Speed Select Technology Executing on CPU model:173[0xad] ------------------------------------------------------------ PCT Capacity from SST-TF bucket-0 ------------------------------------------------------------ ✅ PCT/SST-TF turbo tables detected. PCT_BUCKET=bucket-0 PCT_REPORTING_ANCHORS=4 PCT_ACTIVE_PACKAGES=2 PCT_CORES_PER_PACKAGE=8 PCT_TOTAL_PHYSICAL_CORES=16 PCT_MAX_FREQ_MHZ=4600 PCT_DOMAIN_ANCHORS=pkg0/die0/pd0/cpu0:cores8:freq4600,pkg0/die0/pd1/cpu32:cores8:freq4600,pkg1/die1/pd0/cpu64:cores8:freq4600,pkg1/die1/pd1/cpu96:cores8:freq4600 PCT_PACKAGE_SUMMARY=pkg0:cores8:freq4600:anchors2,pkg1:cores8:freq4600:anchors2 THREADS_PER_CORE=2 PCT_TOTAL_LOGICAL_CPUS=32 ------------------------------------------------------------ Core Power (CLOS) Feature Status ------------------------------------------------------------ ✅ Core Power feature ENABLED ✅ CLOS ENABLED ------------------------------------------------------------ CPU -> CLOS Mapping via get-assoc ------------------------------------------------------------ CLOS distribution (count by clos id): clos:0 -> 32 CPUs clos:2 -> 224 CPUs ------------------------------------------------------------ CPU list for TARGET_CLOS=0 ------------------------------------------------------------ clos:0 CPU list: 0-3,32-35,64-67,96-99,128-131,160-163,192-195,224-227 Wrote clos:0 CPU list to /workspace/benchmarks/results/clos0_cpulist.txt ------------------------------------------------------------ PCT Budget Validation for CLOS0 ------------------------------------------------------------ CLOS0 CPU count : 32 PCT bucket : bucket-0 PCT reporting anchors : 4 PCT active packages/sockets : 2 PCT cores per package/socket : 8 PCT physical core budget : 16 PCT max frequency : 4600 MHz Threads per core : 2 Expected PCT logical CPU budget : 32 ✅ CLOS0 CPU count exactly matches the bucket-0 PCT logical budget. ------------------------------------------------------------ Summary ------------------------------------------------------------ ✅ PCT turbo tables detected ✅ PCT capacity detected: 16 physical HP cores total, 32 logical CPUs with HT=2 Count model: bucket-0 counted once per package/socket, not once per powerdomain anchor. ✅ Core Power enabled ✅ CLOS enabled Done. ```
The check script writes the current target-CLOS CPU list to: ```text ./results/clos0_cpulist.txt ``` For the example above, `clos0_cpulist.txt` contains 32 logical CPUs. With Hyper-Threading enabled, that corresponds to 16 physical PCT cores. ## 3. Set PCT And Assigned HP CPUs This step **activates PCT in practice** by assigning selected HP CPUs to **CLOS0**. The setup script intentionally **overwrites existing BIOS/runtime CLOS settings**: 1. Enable Core Power / CLOS. 2. Move **all online CPUs → `OTHER_CLOS`**. 3. Move selected HP CPUs → `HP_CLOS`. This prevents stale BIOS or previous runtime CLOS assignments from leaving unexpected CPUs in CLOS0. ### Set-Script Behavior
The setup script performs the following actions: - Detects PCT capacity from `intel-speed-select turbo-freq info -l `. - Counts `bucket-0` once per package/socket. - Derives `HP_PER_PACKAGE` from `PCT_CORES_PER_PACKAGE` unless overridden. - Reads the PCT reporting anchors from `PCT_DOMAIN_ANCHORS`. - Dispatches each package's `HP_PER_PACKAGE` physical-core budget across that package's reporting powerdomain anchors. - Selects contiguous physical CPUs starting from each reporting anchor CPU. - Includes Hyper-Threading siblings by default with `INCLUDE_HT=1`. - Assigns: - **Selected HP CPUs → CLOS0** by default - **All remaining CPUs → CLOS2** by default </details> Export the kernel build variables first: ```bash source ./set_kernel_env.sh ``` Run the setup: ```bash docker compose --progress=plain --profile set up --abort-on-container-exit ``` Or test the selection without changing the system: ```bash DRY_RUN=1 docker compose --progress=plain --profile set up --abort-on-container-exit ``` ### Example: Package Capacity Dispatched Across Reporting Powerdomain Anchors
Example results ```bash intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | PCT capacity from SST-TF bucket-0 intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | PCT_BUCKET=bucket-0 intel-speed-select-set-1 | PCT_REPORTING_ANCHORS=4 intel-speed-select-set-1 | PCT_ACTIVE_PACKAGES=2 intel-speed-select-set-1 | PCT_CORES_PER_PACKAGE=8 intel-speed-select-set-1 | PCT_TOTAL_PHYSICAL_CORES=16 intel-speed-select-set-1 | PCT_MAX_FREQ_MHZ=4600 intel-speed-select-set-1 | PCT_DOMAIN_ANCHORS=pkg0/die0/pd0/cpu0:cores8:freq4600,pkg0/die0/pd1/cpu32:cores8:freq4600,pkg1/die1/pd0/cpu64:cores8:freq4600,pkg1/die1/pd1/cpu96:cores8:freq4600 intel-speed-select-set-1 | PCT_PACKAGE_SUMMARY=pkg0:cores8:freq4600:anchors2,pkg1:cores8:freq4600:anchors2 intel-speed-select-set-1 | intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | Config intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | ACTION=set intel-speed-select-set-1 | HP_BUCKET=0 TDP_LEVEL=1 intel-speed-select-set-1 | HP_PER_PACKAGE=8 intel-speed-select-set-1 | INCLUDE_HT=1 intel-speed-select-set-1 | HP_CLOS=0 OTHER_CLOS=2 intel-speed-select-set-1 | DEBUG_MODE=0 DRY_RUN=0 DEBUG_VERBOSE=0 DEBUG_MAP=0 intel-speed-select-set-1 | intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | Powerdomain-anchor HP CPU dispatch intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | package 0: HP_PER_PACKAGE=8, reporting_anchors=2, dispatch_per_anchor=[4, 4] intel-speed-select-set-1 | pkg0/pd0/anchor_cpu0 -> 4 physical cores -> core0:0/128 core1:1/129 core2:2/130 core3:3/131 intel-speed-select-set-1 | pkg0/pd1/anchor_cpu32 -> 4 physical cores -> core32:32/160 core33:33/161 core34:34/162 core35:35/163 intel-speed-select-set-1 | package 1: HP_PER_PACKAGE=8, reporting_anchors=2, dispatch_per_anchor=[4, 4] intel-speed-select-set-1 | pkg1/pd0/anchor_cpu64 -> 4 physical cores -> core64:64/192 core65:65/193 core66:66/194 core67:67/195 intel-speed-select-set-1 | pkg1/pd1/anchor_cpu96 -> 4 physical cores -> core96:96/224 core97:97/225 core98:98/226 core99:99/227 intel-speed-select-set-1 | HP_EFFECTIVE=0-3,32-35,64-67,96-99,128-131,160-163,192-195,224-227 intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | Computed CPU lists intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | HP effective : 0-3,32-35,64-67,96-99,128-131,160-163,192-195,224-227 intel-speed-select-set-1 | HP CPU count : 32 intel-speed-select-set-1 | Non-HP : 4-31,36-63,68-95,100-127,132-159,164-191,196-223,228-255 intel-speed-select-set-1 | intel-speed-select-set-1 | PCT active packages/sockets : 2 intel-speed-select-set-1 | PCT reporting anchors : 4 intel-speed-select-set-1 | PCT cores per package/socket : 8 intel-speed-select-set-1 | PCT physical core budget : 16 intel-speed-select-set-1 | PCT max frequency : 4600 MHz intel-speed-select-set-1 | intel-speed-select-set-1 | Expected HP CPU count for this INCLUDE_HT setting: 32 intel-speed-select-set-1 | intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | Apply CLOS assignments (overwrite existing BIOS/runtime mapping) intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | Setting ALL CPUs -> CLOS2 first intel-speed-select-set-1 | Setting selected HP CPUs -> CLOS0 intel-speed-select-set-1 | Applied. intel-speed-select-set-1 | intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | Verification (concise CPU->CLOS) intel-speed-select-set-1 | ------------------------------------------------------------ intel-speed-select-set-1 | HP list should be clos:0 intel-speed-select-set-1 | cpu-0 clos:0 intel-speed-select-set-1 | cpu-1 clos:0 intel-speed-select-set-1 | … (showing first 2 lines) intel-speed-select-set-1 | intel-speed-select-set-1 | Non-HP list should be clos:2 intel-speed-select-set-1 | cpu-4 clos:2 intel-speed-select-set-1 | cpu-5 clos:2 intel-speed-select-set-1 | … (showing first 2 lines) intel-speed-select-set-1 | intel-speed-select-set-1 | Done. ```
After applying the set flow, run the check flow again. The check output should show: ```text clos:0 -> 32 CPUs clos:0 CPU list: 0-3,32-35,64-67,96-99,128-131,160-163,192-195,224-227 Expected PCT logical CPU budget : 32 ✅ CLOS0 CPU count exactly matches the bucket-0 PCT logical budget. ``` ## 4. Benchmark CLOS0 CPUs With PerfSpect Tool On The Host Use Docker only to configure and verify PCT/CLOS. Run PerfSpect on the host so the frequency benchmark can access host CPU frequency interfaces directly. ### Prerequisites
Details The host benchmark script reads the CPU list generated by the check profile: ```bash ./results/clos0_cpulist.txt ``` Install PerfSpect on the host first: ```bash mkdir -p "${HOME}/tools" cd "${HOME}/tools" wget -qO- https://github.com/intel/PerfSpect/releases/latest/download/perfspect.tgz | tar -xz sudo ln -sf "${HOME}/tools/perfspect/perfspect" /usr/local/bin/perfspect ``` Confirm it is available: ```bash which perfspect perfspect --help | head ```
### Run The Benchmark By using PerfSpect benchmark feature, it generates a diagram of CPU frequency among different number of active CPU cores. The diagram helps us to understand whether PCT cores can reach the right CPU frequency. Run the full flow with [run_host_perfspect_benchmark.sh](/optimization-zone/hardware/priority_core_turbo/run_host_perfspect_benchmark.sh) : ```bash docker compose --progress=plain --profile set up --abort-on-container-exit docker compose --progress=plain --profile check up --abort-on-container-exit ./run_host_perfspect_benchmark.sh ```
Details Default host benchmark command: ```bash sudo taskset -c "${CLOS_CPUS}" perfspect benchmark --speed --frequency --no-summary --output ``` Override the PerfSpect benchmark options with `PERFSPECT_ARGS`: ```bash PERFSPECT_ARGS="--speed --frequency --memory --no-summary" \ ./run_host_perfspect_benchmark.sh ``` </details> ### Analyze Results Benchmark output is written under: ```bash ./results/perfspect_host_clos0_/ ``` The directory includes: ```text clos0_cpulist.txt perfspect_benchmark.log perfspect/ ``` Check Frequency section in HTML file. This is the frequency diagram on Xeon 6776P with PCT on. image This is the expected pattern: small active core counts hold the highest PCT turbo frequency, and frequency gradually steps down as more physical cores become active. ## 5. Debug / Manual Inspection (Optional)
Debug Details This section is useful for **troubleshooting**, **validation**, or **manual experimentation** with Intel® Speed Select and PCT behavior. Start an interactive shell with the required tools installed: ```bash docker compose run --rm intel-speed-select-shell ``` Useful commands: ```bash # Show platform-level Intel Speed Select capability and CPU model information. # Use this first to confirm whether SST-PP, SST-TF, SST-BF, and SST-CP are supported. intel-speed-select --info # Show SST-TF / PCT turbo bucket information for TDP level 1. # This is where bucket-0 high-priority core count and max turbo frequency are reported. # Example: bucket-0 = 8 high-priority physical cores at 4600 MHz. intel-speed-select turbo-freq info -l 1 # Show whether Core Power and CLOS are enabled. # PCT needs Core Power / CLOS enabled before CLOS0 CPU assignment is effective. intel-speed-select core-power info # Show the CLOS association for CPU 0. # This tells which CLOS class CPU 0 currently belongs to, for example clos:0 or clos:2. intel-speed-select -c 0 core-power get-assoc ```