How to Debug Slow CMake Configuration with VCPKG on RHEL8

Slow CMake configuration times with VCPKG on RHEL8 can be frustrating even when VCPKG itself runs quickly. Effective cmake configuration debugging requires a systematic approach combining CMake tracing tools, VCPKG-specific debugging techniques, and system-level monitoring to identify the root cause of performance issues.

Contents

• Understanding Slow CMake Configuration with VCPKG
• CMake Tracing and Profiling Techniques
• VCPKG-Specific Debugging Approaches
• System-Level Performance Monitoring on RHEL8
• Advanced Troubleshooting and Optimization Strategies
• Sources
• Conclusion

Understanding Slow CMake Configuration with VCPKG

Slow CMake configuration with VCPKG can be caused by several factors even when VCPKG itself runs quickly and your vcpkg.json file is empty. Understanding these root causes is the first step in effective cmake configuration debugging.

One common issue is related to how CMake cache variables are set. When variables are set in PARENT_SCOPE instead of CACHE, VCPKG may need to reinitialize on every configuration run. According to a fix documented in the VisualBoyAdvance-M repository, setting CMake cache variables in CACHE instead of PARENT_SCOPE can resolve slow VCPKG startup times.

Another significant bottleneck is how VCPKG handles manifest-mode configurations. As noted in a Microsoft VCPKG issue report, the vcpkg install command gets run for every CMake rerun in manifest-mode, which can cause delays even if no actual installation occurs. This behavior can lead to the 60-100 second delays you’re experiencing despite no visible CPU usage spikes.

Additional potential causes include:

• Expensive find_package() calls in your CMake configuration
• Network-related delays when checking for package availability
• File system operations that are slow on your RHEL8 system
• CMake modules that perform extensive operations during configuration

Understanding these potential causes helps narrow down the debugging process and select the most appropriate diagnostic tools for your specific situation.

CMake Tracing and Profiling Techniques

CMake provides several powerful tracing options that can help identify bottlenecks in your configuration process. While you’ve already tried cmake --trace-expand without success, there are more targeted approaches that can provide deeper insights.

The most effective CMake tracing technique is to use the --trace-source option to focus on specific files. As explained in the HSF Training CMake guide, you can run:

cmake -S . -B build --trace-source=CMakeLists.txt

This will generate detailed trace information only for your main CMakeLists.txt file, making it easier to spot expensive operations. You can also trace specific source files that you suspect might be causing delays.

Another useful option is --trace, which provides a full trace of all CMake operations:

cmake -S . -B build --trace

For more granular output, --trace-expand can be combined with other options:

cmake -S . -B build --trace-expand --trace-format=list

These traces can help identify:

• Time-consuming find_package() operations
• Expensive generator expressions
• Recursive calls that might be inefficient
• File system operations that are causing delays

When analyzing the traces, look for operations that take an unexpectedly long time or appear to be called repeatedly. The traces will show the call stack and timing information, allowing you to pinpoint exactly where the delays are occurring.

For comprehensive cmake configuration debugging, consider running multiple traces with different focus areas to build a complete picture of where time is being spent during the configuration process.

VCPKG-Specific Debugging Approaches

VCPKG provides several debugging options that can help identify performance issues in the integration with CMake. These options are specifically designed to diagnose problems in the VCPKG cmake integration process.

One of the most useful VCPKG debugging techniques is using the experimental --debugger option. According to the Microsoft VCPKG documentation, this option can be used with ports that use vcpkg_cmake_configure to get detailed debugging information.

To enable VCPKG debugging in your CMake configuration, you can add the following flag:

cmake -S . -B build -DVCPKG_INSTALL_OPTIONS="--debug"

Alternatively, you can add this to your CMakePresets.json configuration:

{
 "version": 3,
 "cmakeMinimumRequired": {
 "major": 3,
 "minor": 20,
 "patch": 0
 },
 "configurePresets": [
 {
 "name": "debug",
 "displayName": "Debug Configuration",
 "inherits": "base",
 "cacheVariables": {
 "VCPKG_INSTALL_OPTIONS": "--debug"
 }
 }
 ]
}

The Microsoft VCPKG binary caching troubleshooting guide also suggests enabling additional debug information to diagnose performance issues. This can help identify whether VCPKG is spending time on package resolution, dependency checking, or other operations that might be causing delays.

Another useful approach is to examine the VCPKG_MANIFEST_MODE and VCPKG_OVERLAY_PORTS variables. Even with an empty vcpkg.json file, certain configurations can cause VCPKG to spend significant time on manifest processing or overlay port resolution.

For more detailed analysis, you can also try running VCPKG with increased verbosity:

cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=/path/to/vcpkg/scripts/buildsystems/vcpkg.cmake -DVCPKG_VERBOSE=ON

These VCPKG-specific debugging techniques combined with CMake tracing provide a comprehensive approach to cmake configuration debugging when working with VCPKG on RHEL8.

System-Level Performance Monitoring on RHEL8

When CMake configuration appears to be slow without visible CPU usage spikes, the issue might be at the system level rather than within CMake or VCPKG themselves. RHEL8 provides several performance monitoring tools that can help identify system-level bottlenecks.

One of the most useful tools on RHEL8 is turbostat, which provides detailed information about CPU frequency and power states. As documented in the Red Hat Enterprise Linux 8 documentation, turbostat can help identify if the CPU is in a low-power state that might be causing delays:

turbostat --show CPU Avg_MHz --interval 1

While running this command, execute your CMake configuration and watch for any unexpected changes in CPU frequency. If the CPU frequency remains consistently low during the configuration process, it might explain the slow performance despite no visible CPU usage spikes.

Another valuable tool is iostat, which can reveal disk I/O bottlenecks:

iostat -xz 1

Run this command while performing your CMake configuration to monitor disk utilization. If you see high await or svctm values, it indicates that the system is waiting for disk operations, which could be causing the delays.

RHEL8 also includes perf, a powerful performance analysis tool:

perf record -g -- /usr/bin/cmake -S . -B build
perf report

This will generate a detailed profile of your CMake configuration process, showing exactly where time is being spent at the system level.

For network-related issues, you can use tools like netstat or ss to monitor network connections:

ss -tulnp | grep cmake

These system-level monitoring techniques can often reveal issues that aren’t apparent from within CMake or VCPKG, such as:

• Disk I/O bottlenecks
• CPU frequency scaling issues
• Network latency
• Memory pressure
• File system operations

When combined with CMake tracing and VCPKG debugging, system-level monitoring provides a complete picture for effective cmake configuration debugging on RHEL8.

Advanced Troubleshooting and Optimization Strategies

After implementing the basic debugging techniques, you may need to explore more advanced strategies to identify and resolve the performance issues with CMake configuration using VCPKG on RHEL8.

One advanced approach is to use strace to trace system calls made during CMake configuration:

strace -f -o cmake.strace -T -tt cmake -S . -B build

This will generate a detailed trace of all system calls made by CMake and its child processes, including timing information. Analyzing this trace can reveal expensive system operations that might not be visible through other monitoring tools.

For network-related issues, you can use tcpdump to capture network traffic:

tcpdump -i any -s 0 -w cmake.pcap port 80 or port 443

Run this while performing your CMake configuration, then analyze the capture to identify any unusual network activity or delays.

Another powerful technique is to use CMake’s built-in timing capabilities. You can add the following to your CMakeLists.txt to get detailed timing information for individual functions:

function(time_function FUNCTION_NAME)
 set(START_TIME ${CMAKE_CURRENT_FUNCTION_TIME})
endfunction()

function(log_function_time FUNCTION_NAME)
 math(EXPR ELAPSED_TIME "${CMAKE_CURRENT_FUNCTION_TIME} - ${START_TIME}")
 message(STATUS "Function ${FUNCTION_NAME} took ${ELAPSED_TIME} seconds")
endfunction()

For VCPKG optimization, consider these strategies:

1. Enable binary caching if not already enabled
2. Use vcpkg export to pre-compile packages
3. Consider using vcpkg’s manifest-mode more efficiently
4. Review your toolchain file for any expensive operations

If you’re using a version control system for your project, you may also want to profile different commits to identify when the performance regression was introduced. This can help narrow down the specific changes causing the slowdown.

Finally, consider the environment variables that might affect CMake and VCPKG performance:

• CMAKE_BUILD_PARALLEL_LEVEL
• CMAKE_GENERATOR
• VCPKG_DEFAULT_BINARY_CACHE
• VCPKG_OVERLAY_PORTS
• VCPKG_OVERLAY_TRIPLETS

By systematically applying these advanced troubleshooting strategies, you can often identify and resolve even the most elusive performance issues in cmake configuration debugging with VCPKG on RHEL8.

Sources

1. VisualBoyAdvance-M Debugging Fix — Fix for slow VCPKG startup by setting cache variables in CACHE instead of PARENT_SCOPE: https://github.com/visualboyadvance-m/visualboyadvance-m/commit/bbaf70c083f621c9eac5177eb3084eb75de30cd0
2. Microsoft VCPKG Performance Issues — Explanation of how vcpkg install command runs on every CMake rerun in manifest-mode: https://github.com/microsoft/vcpkg/issues/29857
3. VCPKG Debugging Options — Documentation for experimental --debugger option for ports using vcpkg_cmake_configure: https://learn.microsoft.com/en-us/vcpkg/commands/common-options
4. VCPKG Debugging Techniques — Details about adding -DVCPKG_INSTALL_OPTIONS=“–debug” in CMake configure call: https://learn.microsoft.com/en-us/vcpkg/users/binarycaching-troubleshooting
5. Common CMake Debugging Causes — Comprehensive list of common causes for slow CMake configuration with VCPKG: https://gist.github.com/MangaD/1c062db504b262709f4c85ca6a19b06d
6. CMake Tracing Techniques — Detailed explanation of cmake -S . -B build --trace-source=CMakeLists.txt and other CMake tracing options: https://hsf-training.github.io/hsf-training-cmake-webpage/08-debugging/
7. RHEL8 System Monitoring — Information about turbostat and iostat tools available on RHEL8 for system performance monitoring: https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/8/html/monitoring_and_managing-system-status-and-performance/overview-of-performance-monitoring-options_monitoring-and-managing-system-status-and-performance

Conclusion

Debugging slow CMake configuration times with VCPKG on RHEL8 requires a systematic, multi-faceted approach. Even when VCPKG itself runs quickly and your vcpkg.json file is empty, several factors can cause significant delays in the configuration process.

Effective cmake configuration debugging begins with understanding the potential root causes, from CMake cache variable settings to VCPKG’s manifest-mode behavior. Using CMake tracing techniques like --trace-source and --trace can help pinpoint expensive operations in your configuration files.

VCPKG-specific debugging approaches, including the --debug option and increased verbosity, can reveal issues in the VCPKG integration that might not be apparent otherwise. When combined with system-level monitoring tools available on RHEL8 like turbostat, iostat, and perf, you can get a complete picture of where time is being spent during the configuration process.

For particularly stubborn performance issues, advanced techniques like strace profiling and network traffic analysis can uncover hidden bottlenecks. By systematically applying these debugging strategies and optimization techniques, you can identify and resolve even the most elusive performance issues in your CMake configuration with VCPKG on RHEL8.