GC stack trace

[Vinayaka Kamath]

Hello All,

The project that I am working on embeds v8 in it. One of the users reported seeing the following trace in the log files.

2020-12-01T04:48:31.912-08:00 [INFO] "Starting Function for:" "config::sfdc::accounts"
2020-12-01T04:48:31.912-08:00 [INFO] "Fetching list of:" "SFDC::accounts"
2020-12-01T04:48:31.950-08:00 [INFO] "Fetching SFDC Objects: " "accounts"
2020-12-01T04:54:27.535-08:00 [INFO] "Fetch SFDC (curl) response status:" 200 "/ #Objects:" 22179
2020-12-01T04:54:43.716-08:00 [INFO]
2020-12-01T04:54:43.716-08:00 [INFO] <--- Last few GCs --->
2020-12-01T04:54:43.716-08:00 [INFO]
2020-12-01T04:54:43.716-08:00 [INFO] [5463:0x7f2af005c020] 774469 ms: Mark-sweep 1110.6 (1264.9) -> 1110.6 (1210.4) MB, 497.6 / 0.0 ms (average mu = 0.778, current mu = 0.000) last resort GC in old space requested
2020-12-01T04:54:43.716-08:00 [INFO] [5463:0x7f2af005c020] 775228 ms: Mark-sweep 1110.6 (1210.4) -> 1110.6 (1196.9) MB, 758.3 / 0.0 ms (average mu = 0.616, current mu = 0.000) last resort GC in old space requested
2020-12-01T04:54:43.716-08:00 [INFO]
2020-12-01T04:54:43.716-08:00 [INFO]
2020-12-01T04:54:43.716-08:00 [INFO] <--- JS stacktrace --->
2020-12-01T04:54:43.716-08:00 [INFO]
2020-12-01T04:54:43.716-08:00 [INFO] ==== JS stack trace =========================================
2020-12-01T04:54:43.716-08:00 [INFO]
2020-12-01T04:54:43.716-08:00 [INFO] 0: ExitFrame [pc: 0x7f2b037168ae]
2020-12-01T04:54:43.716-08:00 [INFO] Security context: 0x32a0fa3096b9 <JSObject>
2020-12-01T04:54:43.716-08:00 [INFO] 1: FetchObjects_SFDC(aka FetchObjects_SFDC) [0x32a0fa30ea89] [Data_Ingestion.js:324] [bytecode=0x4ef5172bd09 offset=348](this=0x125eac5825a1 <undefined>,0x19c460402291 <Object map = 0x12664db89e61>)
2020-12-01T04:54:43.716-08:00 [INFO] 2: OnUpdate [0x32a0fa30e951] [Data_Ingestion.js:64] [bytecode=0x4ef5172b1e9 offset=281](this=0x32a0fa30fd81 <JSGlobal Object>,0x19c460402291 <Object map ...

2020-12-01T04:54:43.716-08:00 [INFO]

I do not have much information on what the user was trying to do here. Can someone elaborate on when do we generally see GC traces like these?

Thanks  

[Ben Noordhuis]

The what is that V8 ran out of JS heap memory, the why I can't tell -
maybe FetchObjects_SFDC() allocates a lot, maybe there's a memory leak
in the JS code somewhere.

The default heap limit is approx. 1.4 GB on 64 bits systems. You can
change it with the --max_heap_size=<n> flag, where n is in megabytes.

There's also --max_old_space_size=<n>, that's an older flag mentioned
in many blog posts. It works pretty much the same.

[Vinayaka Kamath]

Thanks a lot Ben, that was really helpful.

The project is using v8 engine embedded in the C++ code, is a build config or can I pass that setting as a parameter in the C++ code? 

[Alex Kodat]

You can use SetFlagsFromString:

V8::SetFlagsFromString("--max_heap_size=4096");  // Pick your value

I didn't look but I would assume you need to do this before you create the Isolate.

There's also the Isolate AddNearHeapLimitCallback method but that's a bit more work to use.