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GuidesGuidesRun the spark profiler while your server lags, then actually read the report it gives you — the call tree, what "% of tick" means, and the culprit patterns you'll recognize.
Fixing Minecraft lag tells you to stop guessing and profile with spark. This guide is the part after that: you've run the profiler, you have a link to a report, and now a wall of nested method names is staring back at you. Here's how to read it and walk away knowing exactly what's eating your tick.
| At a glance | |
|---|---|
| You need | spark installed from the Plugins page (needs a plugin-capable server — Purpur, Paper, Spigot family) |
| Plan | Any — profiling works fine on the free plan |
| Prerequisite | Fixing Minecraft lag for which lag you actually have |
Before the full profiler, spark has two commands that answer "is it even the server?" in seconds. Run them in the Console or in-game:
| Command | What it tells you |
|---|---|
/spark tps |
Current TPS (ticks per second — 20 is healthy) and MSPT (milliseconds per tick), plus CPU. If TPS is 20 and MSPT is low, the server is fine and your lag is client-side or ping |
/spark health |
A broader snapshot — memory use, garbage-collection pauses, disk. Good for spotting a memory problem masquerading as lag |
A tick is one heartbeat of the server; it aims for 20 per second, so it has 50 ms to do everything before it falls behind. When MSPT creeps above 50, TPS drops below 20 and the world visibly slows. That 50 ms budget is the number everything below is measured against.
/spark tps says whether you're slow; the profiler says why. The idea is simple: start it, let the lag happen for a minute or two, stop it, and spark hands you a shareable online report.
/spark profiler
Start it, reproduce the lag (or just wait if it's constant), then stop it. spark uploads the result and prints a link you open in a browser.
⚠️ Heads up: The exact profiler flags and subcommands (timeouts, thread selection,
start/stopvs--stop) have changed between spark versions. Don't memorize a flag off an old video — run/spark profilerand read what your build prints, or check the current reference at spark.lucko.me/docs.
Open the link and you'll see the sampler — a tree of method calls, each with a percentage. This is a call tree: spark took thousands of snapshots of what the server was doing, and the tree shows where the time went, most-expensive branches first.
Read it like this:
com.example.plugin…), a world-gen routine, an entity-tick method. That's your culprit.Every frame in the tree shows a percentage — say "42%". That is the share of the sampled time that this method (and everything it called) was on the stack. It is not 42% of one tick; it's 42% of the whole profiling window spent in that code path. A branch sitting at 40%+ is doing a lot of your server's work, so that's where fixing something buys you the most back.
🎯 Good to know: Percentages are relative to what the server was doing, not to a fixed budget. On an idle server the biggest branch might still be tiny in real terms. Cross-check with MSPT from
/spark tps— high percentage and high MSPT together is what points at a real problem.
After a few reports the same shapes turn up. Here's what a fat branch usually means:
| What dominates the tree | Usually means | First move |
|---|---|---|
| An entity / mob tick method | Too many mobs, or heavy AI/pathfinding (mob farms, big herds) | Cap mob counts, thin out farms; lower simulation-distance |
| Chunk generation / world loading | Players exploring into fresh terrain | Pre-generate the world; lower view-distance |
| A specific plugin's package | That plugin is doing expensive work each tick | Check its config, update it, or replace it — see diagnosing plugin conflicts |
| Redstone / hopper / block-entity ticks | A big redstone contraption or hopper array | Optimize the build; some forks (Purpur) have hopper-tuning options |
Garbage collection high in /spark health |
Memory pressure, not CPU work | Confirm with Out of memory — don't tune the wrong thing |
The value of the report is that it turns "the server is laggy" into "this named thing is spending this much time." You fix that and re-profile — instead of flipping settings at random and hoping.
Make one change, restart, and run /spark tps again with the same load. Better MSPT and TPS closer to 20 means it worked. For a real before/after, profile again and confirm the branch that used to dominate has shrunk. One change at a time, always — so you know which one helped.
/spark does nothing — spark isn't installed or didn't load. Install it from the Plugins page, restart, and check it appears in /plugins green.