Hotspot turns a Linux perf.data recording into an interactive graphical profile. Record the workload with perf, open the resulting file in Hotspot, then use its timeline, flame graphs, call trees and disassembler to investigate where sampled CPU time went. It makes recorded data easier to navigate; it cannot recover stacks, symbols or event data that were not captured or are unavailable.
What Hotspot does—and what it does not
KDAB describes Hotspot’s main feature as “the graphical visualization of a perf.data file.” It is a standalone GUI for Linux perf recordings, not a replacement recording engine. You collect samples with the system’s perf command, then inspect them in Hotspot. The project’s README documents the interface and its current limitations.
That division matters: a viewer can reorganize and explain the data in a profile, but it cannot recreate source lines, call paths or event information that the recording did not capture. Keep perf report available for modes Hotspot does not support.
Record a profile that can answer your question
Choose a repeatable workload and record the part you want to understand. KDAB’s example for collecting call stacks is:
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perf record --call-graph dwarf <your application>
Replace <your application> with the program and its arguments. By default, perf writes perf.data in the current directory. DWARF unwinding can provide call stacks, but the resulting profile depends on available unwind information and can be larger or more expensive to collect than a profile without call graphs.
For especially deep stacks that are being truncated, KDAB suggests increasing the DWARF stack dump size:
perf record --call-graph dwarf,32768 <your application>
This can make the data file grow dramatically. Use it when stack depth is the problem, not as a default setting. If you need branch-stack analysis, the branch information must also be recorded: Linux perf documents collection with perf record -b or an appropriate branch filter. Branch-history analysis likewise depends on branch data and call graphs; a GUI cannot infer either afterward. See the Linux perf report documentation.
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Open the recording in Hotspot
- Run
perf recordfrom the workload’s directory, or note the path where the recording is written. - Open the current directory’s default recording by launching
hotspot, or open a specific recording withhotspot /path/to/perf.data. - If the profile came from an embedded system, analyze it on a development machine and configure Hotspot to find the target’s matching executable, libraries and debug information. Use the relevant path options—
--sysroot,--appPath,--extraLibPathsand--debugPaths—when the target’s file paths do not match the analysis machine. Check the installed version’s help or man page for exact option syntax.
Hotspot’s command-line options are also listed in the Debian testing Hotspot man page; availability and option details can vary by package version.
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Narrow the profile with the timeline
Use the timeline to select a time range, process or thread. Hotspot updates its data views to reflect that selection, letting you focus on a specific phase or execution context rather than treating the entire recording as one undifferentiated result.
Follow call paths with flame graphs and call trees
Flame graphs and top-down or bottom-up call-tree views help connect costly functions to the paths that called them. Start with a broad view to locate a concentrated cost, then follow the callers or callees to see whether the expense belongs to the function itself or to work deeper in its call path. If names or paths are absent, first check that the recording has usable call stacks and that Hotspot can find the matching binaries and symbols.
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Inspect instruction-level cost
Hotspot’s disassembler displays cost per instruction and can link assembly instructions to corresponding source lines when the source and debug information are available. This is useful after a profile points to a specific function and you need to see where its instruction-level cost accumulates; it does not guarantee source context for every binary.
Why function names, source lines or backtraces may be missing
Readable stacks depend on successful unwinding and on Hotspot locating the correct executable and libraries. Debug information is needed for source-level context. KDAB notes that missing ELF files or debug information can break backtraces, while stacks that exceed the recorded depth can be truncated. For remote or embedded profiles, configure the paths to the target’s matching files rather than relying on the analysis machine’s unrelated copies.
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- Broken function names or backtraces: verify that the matching executable and libraries are present and accessible.
- Truncated deep stacks: consider a larger DWARF stack dump size, understanding that this can dramatically increase file size.
- No source correspondence: make the relevant source and debug files available and configure their paths; assembly cost can still be inspected without source mapping.
KDAB says Hotspot’s unwinding relies on perfparser and elfutils libdw. The project also advises against running Hotspot as root. If recording is blocked by perf permissions, investigate the system’s perf-event policy and the privilege handling for the installed tools rather than assuming the GUI needs to run with elevated privileges. See the project README for its troubleshooting notes.
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Off-CPU profiling and advanced perf modes
Hotspot release notes document off-CPU support in version 1.2.0. KDAB gives this recording pattern:
perf record --call-graph dwarf -e cycles -e sched:sched_switch --switch-events --sample-cpu ...
The ellipsis represents workload-specific arguments; this is a pattern, not a universal copy-and-run command. Confirm that the installed Hotspot version, kernel and distribution support the relevant events and options before using it. The feature’s release reference is in the Hotspot release notes.
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Hotspot does not implement every advanced perf report option. Its README identifies --itrace, --mem-mode, --branch-stack and --branch-history as unsupported. For those workflows, use perf report or another tool that supports the required data mode. The Linux perf branch-analysis requirements also mean the relevant branch data must be captured at recording time.
Exporting data for later analysis
Hotspot can export analyzed data in its .perfparser format, which can make a profile self-contained for later use. KDAB cautions that the format is not stable across Hotspot versions and that interoperability with other visualization tools is limited. Treat it as a convenience for compatible Hotspot workflows, not as a durable interchange format or a substitute for retaining the original perf.data.
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