[GHC] #14886: Add max GC pause to GHC.Stats/-t --machine-readable

[GHC]

#14886: Add max GC pause to GHC.Stats/-t --machine-readable
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           Reporter:  Fuuzetsu       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Sometimes we're interested in latency of our programs. Often the upper
 highest percentiles are by garbage collector pauses due to stop-the-world
 implementation.

 It seems that currently the only way to see the highest pause is to use
 the {{{-s}}} flag or alternatively {{{-S}}}. This is problematic in two
 scenarios:

 1. You don't have a human at the monitor parsing the {{{-s}}} output.
 2. You are unable to get the maximum pause statistic programmatically
 inside the program itself.

 We do have {{{-t -machine-readable}}} which is nearly what I'd want but
 missing the actual pause information. Either that should include all the
 extra info from {{{-s}}} or {{{-s}}} should have machine-readable output
 format too.

 There is the {{{-S}}} option which will print garbage collections as they
 happen. We could parse and manually track the highest pause time but it
 requires an external process and a parser. If you want to get that
 information back into the process then you have to do even more work.

 {{{GHC.Stats}}} is of no help. It does not track or expose pause values.
 Even the "last GC info" APIs don't provide this (and they are not a
 solution anyway as you can only get info on last GC).

 I would therefore like to request two similar features:

 * Add relevant information from {{{-s}}} to {{{-t}}} machine readable
 output or add machine readable format option to {{{-s}}}.
 * Add maximum pause information to GHC.Stats interface.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14886>
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