Reporting of gc+stats

[Justinas Dabravolskas]

Hi,
hope this is the right place to ask  a few more questions. Sorry if answers were simply overlooked.  
1. I can't find an option for periodic printing of pacer statistics.  (gc+stats prints just on exit?)
Pacers activity looks like the first indication of gc pressure. Don't you think it would be useful to get this information from gc log asap?  
Maybe you can introduce ShenandoahPacerStatsPeriod flag for periodic printing of _delays histogram?
1. How to interpret breakdown of roots scanning? 
e.g.: "Scan Roots" took 327s, but I can't figure out what part exactly from breakdown. What I am missing?
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ] Pause Init Mark (N)         =   338.46 s (a =    51057 us) (n =  6629) (lvls, us =     4023,    45508,    50781,    56641,    88198)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]   Accumulate Stats          =     1.42 s (a =      214 us) (n =  6629) (lvls, us =       33,      182,      209,      236,     3329)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]   Make Parsable             =     6.10 s (a =      920 us) (n =  6629) (lvls, us =       55,      732,      891,     1074,     6075)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]   Clear Liveness            =     1.29 s (a =      195 us) (n =  6629) (lvls, us =       91,      168,      189,      211,     3251)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]   Scan Roots                =   327.02 s (a =    49332 us) (n =  6629) (lvls, us =     3613,    43945,    49023,    54688,    86252)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: Thread Roots         =     6.68 s (a =     1008 us) (n =  6629) (lvls, us =      137,      869,      994,     1113,     9622)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: Universe Roots       =     0.08 s (a =       12 us) (n =  6629) (lvls, us =        2,        8,        9,       13,     4443)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: JNI Roots            =     0.37 s (a =       55 us) (n =  6629) (lvls, us =        4,       37,       46,       59,     8601)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: Synchronizer Roots   =     0.00 s (a =        1 us) (n =  6629) (lvls, us =        0,        0,        0,        1,       22)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: Management Roots     =     0.06 s (a =        8 us) (n =  6629) (lvls, us =        1,        3,        8,       11,      102)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: System Dict Roots    =     0.54 s (a =       81 us) (n =  6629) (lvls, us =       16,       61,       74,       89,     4439)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: CLDG Roots           =    80.66 s (a =    12167 us) (n =  6629) (lvls, us =      811,     9004,    11133,    14453,    27327)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]     S: JVMTI Roots          =     0.50 s (a =       75 us) (n =  6629) (lvls, us =        7,       48,       62,       79,    12281)
[2020-01-21T07:33:24.757-0500][info ][gc,stats     ]   Resize TLABs              =     0.95 s (a =      143 us) (n =  6629) (lvls, us =        7,      104,      133,      172,     1513)

Regards,
Justinas