In jstat, YGC and FGC count young and full garbage-collection events; YGCT, FGCT, and GCT are cumulative time spent on those collections. Compare consecutive samples to see whether an event occurred between them. Use -gcutil for utilization percentages, -gc for capacities and usage in KB, and -gccause to include reported collection causes.
What jstat reports
Oracle describes jstat as a command that displays performance statistics for an instrumented Java HotSpot VM. The fields discussed here are from the Java SE 17 reference; available fields and their semantics can vary with the target JDK and collector, so consult the man page for the JDK you are monitoring. Oracle Java SE 17 jstat reference.
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For percentage-oriented samples, run jstat -gcutil <vmid> <interval-ms> <count>. For capacity and usage values in KB, use jstat -gc <vmid> <interval-ms> <count>. In both outputs, the GC counters and times are cumulative.
Percentage fields in -gcutil
S0andS1: survivor-space utilization percentages.E: eden utilization percentage.O: old-space utilization percentage.M: metaspace utilization percentage.CCS: compressed-class-space utilization percentage.
Capacity and usage fields in -gc
S0C/S1CandS0U/S1U: survivor-space capacities and usage.EC/EU: eden capacity and usage.OC/OU: old-space capacity and usage.MC/MU: metaspace committed size and utilization.CCSC/CCSU: compressed-class-space committed size and used space.
The -gc output also includes YGC, YGCT, FGC, FGCT, and GCT.
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How to identify a collection between samples
Read adjacent rows together. A counter increase means the corresponding event occurred during the interval between those samples; the difference in the time counter is the additional cumulative GC time recorded over that interval.
YGCincreases: one or more young-generation collections occurred. The change inYGCTis the added young-GC time.FGCincreases: one or more full collections occurred. The change inFGCTis the added full-GC time.GCTis cumulative total GC time; subtract adjacent values to see the added total time over the interval.
For example, if YGC rises from 7 to 8 and YGCT rises from 0.300 to 0.378 seconds, one young collection occurred between those rows and the cumulative young-GC time increased by 0.078 seconds. These are illustrative values from Oracle’s Java SE 17 example, not a performance target or threshold.
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What -gccause adds
Run jstat -gccause <vmid> <interval-ms> <count> to get the -gcutil summary plus two cause fields: LGCC, the cause of the last GC, and GCC, the cause of the current GC when applicable. Causes can help explain why a collection was reported, but interpret them alongside counter changes and occupancy; a cause field does not replace the event counts or time deltas.
Why old-space utilization can rise after a young collection
A young collection can reduce eden utilization, change survivor-space occupancy, and promote surviving objects into old space. Promotion is why old-space utilization may rise across a young-GC event rather than fall. In Oracle’s Java SE 17 worked example, eden rises from 70.31% to 96.53%, then falls to 1.98% after a collection; YGC advances from 7 to 8, YGCT from 0.300 to 0.378 seconds, survivor utilization changes, and old-space utilization rises from 66.80% to 68.19%. This illustrates one run and collector, not a universal pattern or evidence by itself of a problem. Oracle’s jstat examples.
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Useful sampling controls
A typical command specifies the VM identifier followed by an optional sampling interval in milliseconds and count. For example, jstat -gcutil 21891 250 7 samples the VM identified as 21891 at 250-millisecond intervals for seven samples.
- Add
-tto prepend a timestamp to each sample. - Add
-h Nto repeat the column header every N lines, which can make longer output easier to read.
When new-generation detail matters
Use jstat -gcnew <vmid> <interval-ms> <count> when you need additional young-generation details. Alongside survivor and eden capacities and usage, it reports YGC and YGCT plus:
TT: current tenuring threshold.MTT: maximum tenuring threshold.DSS: desired survivor size.
Oracle’s example shows TT falling when survivor demand would exceed DSS, then returning when survivor pressure is low. That example describes a particular collector and run; it is not a universal threshold rule.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare two runs fairly
For a useful comparison, keep the sampling interval, JDK version, collector, and workload constant. Then compare the same signals rather than isolated end-of-run values:
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- Event frequency: compare changes in YGC and FGC over the same observation period.
- Time accumulation: compare YGCT, FGCT, and GCT deltas over that period.
- Post-event occupancy: inspect E, O, S0, and S1 in rows after counter changes.
- Reported causes: review LGCC and GCC from
-gccause, where available.
A difference in counters or occupancy is a clue to investigate, not by itself proof that a configuration change caused a performance improvement or regression.
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