Specific meanings of jstat parameters : YGCT FGCT GCT

Question

I need to use jstat to measure some GC parameters of a program. Jstat provides set of parameters ( S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT) From those I am bit confused with the descriptions for YGCT, FGCT and GCT.

(YGCT Young generation garbage collection time.
FGCT Full garbage collection time.
GCT Total garbage collection time.)

I have 2 questions.

1) What does these three parameters (YGCT, FGCT and GCT) actually measure? A small comparison would be very helpful

2) I how can I know the unit of time that they are mentioning? (milliseconds/ seconds/.... )

I referred many documentations including

Interpreting jstat results
http://www.cubrid.org/blog/dev-platform/how-to-monitor-java-garbage-collection/
http://docs.oracle.com/javase/6/docs/technotes/tools/share/jstat.html#output_options
But these does not answer my real question.
Could anyone how has experienced with jstat help me with this?

Thank you.

Jugglery

Answer 1

• YGCT - Seconds spent in young generation collections since the JVM started
• FGCT - Seconds spent doing full garbage collections since the JVM started
• GCT - the sum of the above two values

This knowledge comes from experience and testing - I have not found a good reference defining the time unit and exact meaning.

Rabe

Thank you very much for the clarification. This is exactly what I wanted to know. – Jugglery

Answer 2

    S0C – Current survivor space 0 capacity (KB).
    S1C  –  Current survivor space 1 capacity (KB).
    S0U – Survivor space 0 utilization (KB).
    S1U  – Survivor space 1 utilization (KB).
    EC    – Current eden space capacity (KB).
    EU    – Eden space utilization (KB).
    OC    – Current old space capacity (KB).
    OU    – Old space utilization (KB).
    PC     – Current permanent space capacity (KB).
    PU     – Permanent space utilization (KB).
    YGC   – Number of young generation GC Events.
    YGCT – Young generation garbage collection time.
    FGC   – Number of full GC events.
    FGCT – Full garbage collection time.
    GCT   – Total garbage collection time.

FGCT +YGCT = GCT
FGCT /FGC = avg time taken per full gc cycle 
YGCT / YGC = avg time taken per each young GC

these are useful whenyou are dealing with GC stats.

Automatism