CPU Activity Monitor

A lightweight native Windows app (C++17, Win32/GDI) that shows CPU usage per logical core as a scrolling bar-chart history. Readings come from the Performance Data Helper (PDH) API; rendering is done with GDI (double buffered via a memory DC, no flicker). The whole thing compiles down to a single ActivityMonitor.exe under 48 KiB - no runtime DLLs beyond the standard Windows system libraries - and CI fails the build if that ever regresses (see .github/workflows/ci.yml).

Download

Prebuilt binaries are published on the Releases page for every version tag (vX.Y.Z), as a zip containing ActivityMonitor.exe plus a .sha256 checksum file. See Continuous integration below for how these are built.

Caution

ActivityMonitor.exe is not code-signed, so Windows will show a "Windows protected your PC" / Microsoft Defender SmartScreen warning the first time you run it on another machine. This is expected for a small, unsigned freeware tool - not a sign of tampering. Verify the download's integrity with the .sha256 file if you want to be sure, then click More info -> Run anyway to start it. A proper code-signing certificate (or a free one via SignPath Foundation) is more than this project currently justifies, given it has no significant user base yet.

How it works

• CpuMonitor (src/CpuMonitor.h/.cpp) opens one PDH counter per logical core (\Processor(N)\% Processor Time) plus one for _Total, and collects a new sample once per second. The last 300 values per core (5 minutes at 1 Hz) are kept in a ring buffer.
• GraphWindow (src/GraphWindow.h/.cpp) is the main window. It lays out the cores automatically in a roughly square grid and draws one panel per core with a title, the current percentage, gridlines (0/25/50/75/100%), and the usage history as a bar chart. Bars have a fixed pixel width and gap regardless of panel size - resizing the window changes how many bars fit, never how wide they are - and each bar is filled with a tiled "segment" pattern brush so it renders as a stack of discrete lit blocks (LED-matrix style) instead of a solid area. Everything is drawn into an offscreen bitmap (memory DC) and only then blitted to the screen via BitBlt.
• The title bar additionally shows the current overall CPU usage.
• The color theme is light green on dark green (kBarColor / kBackgroundColor / kPanelBgColor in GraphWindow.cpp).
• Panel titles and percentages use a monospaced, "technical"-looking font. GraphWindow::CreateMonospaceFont tries a list of candidates in order (IBM Plex Mono, Cascadia Mono, Consolas, Courier New) and picks the first one actually installed on the system - install IBM Plex Mono for the intended look, otherwise it falls back gracefully.
• The app icon (app.ico, a small bar-chart glyph matching the on-screen theme) is generated by tools/make_icon.ps1.
• A system tray icon shows a live gauge of the overall (all-core average) CPU usage, redrawn every second in the same light-green-on-dark-green style. Closing the window (or minimizing it) hides it to the tray instead of exiting; left-click the tray icon to bring the window back, right-click for a "Show" / "Exit" menu.

Build

Prerequisite: Visual Studio (with "Desktop development with C++") or the Build Tools, plus CMake >= 3.16.

From an "x64 Native Tools Command Prompt for VS" (or any PowerShell/shell that has cl.exe on PATH):

cmake -S . -B build -G "Visual Studio 17 2022" -A x64
cmake --build build --config Release

The resulting ActivityMonitor.exe ends up in build/Release/.

Or with Ninja:

cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build build

MinGW-w64 also works (pdh/gdi32/user32/comctl32 are linked via target_link_libraries):

cmake -S . -B build -G "MinGW Makefiles"
cmake --build build

Continuous integration

Two GitHub Actions workflows live in .github/workflows/:

• ci.yml runs on every push and pull request against main: configures and builds a Release binary on windows-latest, fails the build if ActivityMonitor.exe exceeds the 48 KiB size budget, and uploads it as a build artifact for inspection.
• release.yml runs only when a tag matching vX.Y.Z (e.g. v1.0.0) is pushed. It builds a Release binary with that version baked into the executable's file properties (see src/version.h.in / src/resource.rc), re-checks the size budget, zips the exe together with the README, computes a SHA-256 checksum, and publishes both as a GitHub Release with auto-generated release notes.

To cut a release:

git tag v1.0.0
git push origin v1.0.0

Customization

• Update interval: GraphWindow::kUpdateIntervalMs (default 1000 ms).
• History length: CpuMonitor::kHistoryLength (default 300 samples).
• Bar size: kBarWidth / kBarGap in GraphWindow.cpp (fixed pixel size, independent of window size).
• Segment size: kSegmentLitHeight / kSegmentGapHeight in GraphWindow.cpp.
• Colors: kBarColor, kBackgroundColor, kPanelBgColor, kGridColor, kTextColor, kMutedTextColor in GraphWindow.cpp.
• Font candidates: the list in GraphWindow::CreateMonospaceFont.

License

MIT - see LICENSE.