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Anyone knows where I can read about the output of the --trace-gc option in nodejs?
I am not asking for an explanation on how the gc works, as there is plenty of information about it, just for the output of the --trace-gc.
I can guess what is the meaning of some of the fields but I have no idea of some others.
For instance :
An example :
[12994] 77042 ms: Scavenge 260.7 (298.1) -> 247.7 (298.1) MB, 9.4 ms [allocation failure].
[12994] 77188 ms: Scavenge 261.7 (298.1) -> 249.0 (300.1) MB, 7.4 ms [allocation failure].
[12994] 77391 ms: Scavenge 263.8 (301.1) -> 250.6 (302.1) MB, 8.1 ms [allocation failure].
[12994] 77511 ms: Scavenge 264.8 (302.1) -> 251.8 (304.1) MB, 7.4 ms [allocation failure].
[12994] 77839 ms: Scavenge 273.4 (304.1) -> 260.7 (305.1) MB, 8.3 ms (+ 55.7 ms in 201 steps since last GC) [allocation failure].
[12994] 78052 ms: Scavenge 274.3 (305.1) -> 261.9 (307.1) MB, 8.2 ms (+ 54.4 ms in 192 steps since last GC) [allocation failure].
[12994] 78907 ms: Scavenge 277.3 (308.1) -> 264.2 (309.1) MB, 10.1 ms (+ 51.5 ms in 196 steps since last GC) [allocation failure].
[12994] 80246 ms: Mark-sweep 272.2 (310.1) -> 82.9 (310.1) MB, 45.2 ms (+ 195.4 ms in 690 steps since start of marking, biggest step 1.2 ms) [GC interrupt] [GC in old space requested].
[12994] 80868 ms: Scavenge 99.3 (310.1) -> 85.5 (310.1) MB, 6.5 ms [allocation failure].
[12994] 81039 ms: Scavenge 100.2 (310.1) -> 86.8 (310.1) MB, 6.9 ms [allocation failure].
[12994] 81455 ms: Scavenge 102.2 (310.1) -> 88.8 (310.1) MB, 5.5 ms [allocation failure].
UPDATE
Looking at the file that creates the output (as suggesteed by mtth), I am adding an explanation of all the fields in case anyone is interested :
[12994] 77042 ms: Scavenge 260.7 (298.1) -> 247.7 (298.1) MB, 9.4 ms [allocation failure].
[pid] <time_since_start> :
<Phase> <heap_used_before (old+young)> (<allocated_heap_before>) ->
<heap_used_after (old+young)> (<allocated_heap_after>) MB,
<time_spent_gc> [<reason_of_gc>]
Additionally when there has been any incremental marking between old space gcs (full), it appears in the scavenging trace, like this :
(+ <incremental_time_duration> ms in <incremental_marking_steps> steps since last GC)
When the trace corresponds to an old space gcs (full), it also shows the biggest step duration.
This traces correspond to nodejs 0.12.9, and they look alike at least in nodejs 4.2.2
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Luckily for you, Node. js comes with a garbage collector, and you don't need to manually manage memory allocation.
This small GC is triggered by a number of things (including some heuristics) that can make it difficult to determine precisely what triggered it, but every couple seconds is nothing to be concerned about in practice since it's only blocking for 1-10ms. Under high load applications it will actually run much more often.
Some high-level languages, such as JavaScript, utilize a form of automatic memory management known as garbage collection (GC). The purpose of a garbage collector is to monitor memory allocation and determine when a block of allocated memory is no longer needed and reclaim it.
Scavenge operations occur only with the gencon garbage collection policy. A scavenge operation runs when the allocate space within the nursery area is filled. During a scavenge, reachable objects are copied either into the survivor space within the nursery, or into the tenure space if they have reached the tenure age.
The closest to documentation I could find is the source of the function that generates the output. Using the comments in gc-tracer.h, we can figure out what each entry means. For example:
- what is the meaning of the number in parenthesis
The number inside the parens represents the total memory allocated from the OS (and the one before is the total memory used for objects in the heap).
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