I have gctrace output that looks like this:
gc 6 @48.155s 15%: 0.093+12360+0.32 ms clock, 0.18+7720/21356/3615+0.65 ms cpu, 11039->13278->6876 MB, 14183 MB goal, 8 P
I am not sure how to read the CPU times in particular. I understand that it is broken down into three phases (STW sweep termination, concurrent mark/scan, and STW mark termination), but I'm not sure what the + signs mean (i.e. 0.18+7720 and 3615+0.65). What do these + signs signify?
In your case, they look like assist and termination times;
// CPU time
0.18 : **STW** Sweep termination.
7720ms : Mark/Scan - Assist Time (GC performed in line with allocation).
21356ms : Mark/Scan - Background GC time.
3615ms : Mark/Scan - Idle GC time.
0.65ms : **STW** Mark termination.
I think it changes (or it may) over various Go versions and you can find more detailed info at runtime package docs.
Currently, it is:
gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # P
where the fields are as follows:
gc # the GC number, incremented at each GC
@#s time in seconds since program start
#% percentage of time spent in GC since program start
#+...+# wall-clock/CPU times for the phases of the GC
#->#-># MB heap size at GC start, at GC end, and live heap
# MB goal goal heap size
# P number of processors used
Example here
See also Interpreting GC trace output
gc 6 @48.155s 15%: 0.093+12360+0.32 ms clock,
0.18+7720/21356/3615+0.65 ms cpu, 11039->13278->6876 MB, 14183 MB goal, 8 P