What does `go tool pprof -http=:8080 cpu.pprof` open, and how do you read its flame graph?

## What the flag does `go tool pprof -http=:8080 cpu.pprof` skips the interactive text prompt, starts an HTTP server bound to that address on the local machine, and opens a browser at it. The profile is the same one the text commands read; only the presentation differs, and every text command has a counterpart in the UI. The View menu offers: - **Top** — the same ranking as the `top` command, sortable by clicking a column. - **Graph** — the call graph as a dot-rendered diagram, node size and edge thickness proportional to weight. This rendering requires Graphviz to be installed; so do the `web` and `svg` commands at the text prompt, which is why they fail with a `dot` execution error on a machine without it. - **Flame Graph** — the stacked call-path view described below. - **Peek** — callers and callees, as the `peek` command prints them. - **Source** — annotated source, as `list` prints it, subject to the same requirement that the source files be findable locally. - **Disassemble** — per-instruction weights, which needs the matching binary. The search box at the top applies a focus regexp to every view at once, and the Refine menu offers focus, ignore, hide and show. Filters persist as you move between views, which is the main workflow advantage over typing commands one at a time. ## Reading a flame graph correctly Each box is a frame in a call stack. Boxes stack in the direction of call depth, so a child box sits directly against its caller. **The width of a box is the share of samples in which that frame appeared on the stack at that position** — cumulative weight, the same quantity the cum column reports. Three consequences follow, and each of them is a common misreading: 1. **The horizontal axis is not time.** Siblings are laid out in a stable order so the picture is comparable between runs; nothing about left or right means earlier or later. A flame graph cannot tell you that one phase ran before another. (The tool that shows events on a real timeline is a different one entirely.) 2. **Width is not self time.** Every ancestor of a hot leaf is exactly as wide as the leaf beneath it, plus its siblings. The root frames span nearly the whole picture and are never the answer. 3. **The thing you are looking for is a wide box with no wide child** — a plateau. That is a frame whose width is its own work rather than its callees'. Visually it is a wide, flat top; that is where the self time is, and it corresponds to a high flat value in `top`. Clicking a box zooms so that box becomes the full width, which is how you explore a subtree that is only 5% of the profile but is the part you own. There is a reset control to zoom back out. ## When the graphical view earns its keep The text `top` collapses a function into one row summed over every caller. A flame graph keeps the call paths separate, so a helper invoked from three subsystems appears as three boxes, and you can immediately see that only one of the paths is expensive. That is the single strongest reason to open the browser UI: cost that is *split by context* is hard to read in a flat ranking and obvious in the graph. The converse is also true. A profile whose cost is spread across hundreds of narrow towers is easier to judge from `top` and its sum% column than from a picture, and a picture invites the eye toward the biggest shape, which is exactly the root frame you must ignore. ## Practical notes Bind it to localhost, not a public interface — the UI serves whatever is in the profile, including function and file names, and it is a diagnostic surface, not a service. `-http=:0` picks a free port. The saved profile artifact is all the UI needs; you can browse a profile captured on another host weeks earlier, though the Source view will stay empty unless the matching sources resolve locally. Go 1.26 changed the UI's default landing view to the flame graph; before that it opened on the graph view. Both views have been available for many releases, so the change is which one you see first, not what is available.