[GHC] #15402: The settings and behaviour of idle GC are very confusing

[GHC]

#15402: The settings and behaviour of idle GC are very confusing
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           Reporter:  nh2            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Runtime        |           Version:  8.4.3
  System                             |
           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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 This extracting the issue between comments

 * https://ghc.haskell.org/trac/ghc/ticket/13497#comment:3 and
 * https://ghc.haskell.org/trac/ghc/ticket/13497#comment:14

 into their own issue.

 There are a few problems problems (as of GHC 8.4.3):

 **1) You can't disable idle GC.**

 * This is contrary to the
 [https://downloads.haskell.org/~ghc/8.4.3/docs/html/users_guide/runtime_control.html
 #rts-flag--I%20%E2%9F%A8seconds%E2%9F%A9 manual], which says `Specifying
 -I0 disables the idle GC.`.
 * Passing `+RTS -I0` will run the idle GC a few times and then it will
 stop. [https://ghc.haskell.org/trac/ghc/ticket/13497#comment:10 Detailed
 here].
 * It will run in intervals of `0.3` seconds (which is claimed to be the
 default for `-I` in the manual).

 **2) Giving the default for `+RTS -I` explicitly, namely `+RTS -I0.3`,
 changes the behaviour.**

 * With it given, idle GC will run every 0.3 seconds.
 * With it not given, it will run for a few times at 0.3 second and then
 stop.
 * This is exactly the behaviour of `-I0`. So if the manual said "the
 default for `-I` is `-I0`, that would be (perversely) more accurate.

 **3) The docs suggest idle GC is only in effect with `-threaded`.**

 *
 [https://downloads.haskell.org/~ghc/8.4.3/docs/html/users_guide/runtime_control.html
 #rts-flag--I%20%E2%9F%A8seconds%E2%9F%A9 manual "In the threaded and SMP
 versions of the RTS (see -threaded ...)."]
 * But idle GC settings like `+RTS -I` clearly also affect the nonthreaded
 RTS.

 **4) The concept of running the idle GC "for a little while" is a bit
 dubious in my opinion.**

 * Its [https://ghc.haskell.org/trac/ghc/ticket/13497#comment:13 origin as
 a power-saving mechanism] is sensible.
 * But it makes things "weird": When you look at `strace`, then first
 there's `SIGVTALRM` going on (because the idle GC timer of 0.3 seconds is
 checked against in each timer signal tick of 0.02 seconds by default),
 then after some time they suddenly stop. Of course `SIGVTALRM` occurring
 can also have effects on underlying library calls (syscalls return with
 `EINTR`), so (at least in non`-threaded`) a simple idling Haskell program
 creates some time-dependent behaviour on C libraries being used.
 * This makes debugging signal issues and behaviour extra hard, if you
 constantly have to keep in mind "oh but in the next 0.3 seconds the
 program will behave like this, and then behaviour will change. When you're
 debugging with `strace`, you have to be constantly aware whether or not
 the current output you read is inside or outside that time window.
 * But I don't have an idea of how this could be improved without
 sacrificing something else, so I guess this point just remains as "weird"
 for now.

 ----

 I think points 1-3 are fixable, we should perhaps

 * make `-I<thedefaultvalue>` behave the same as not giving `-I`
 * change the "default value" to something that reflects what the current
 default behaviour actually is
   * the actual possibilities for this option expressed in Haskell would be
 `data IdleGcMode = NoneAtAll | StoppingAfterSeconds Double |
 IntervalSeconds Double`
   * so perhaps we should change `-I` to allow passing the following values
 to it:
     * `-I<seconds>`, e.g. `-I0.3` to mean `IntervalSeconds 0.3`
     * `-I<seconds>stopping`, e.g. `-I0.3stopping` to mean
 `StoppingAfterSeconds 0.3`
     * `-I0` to mean `NoneAtAll`
   * and the default would be `-I0.3stopping` to reflect the current
 default.
 * fix the documentation to reflect all this

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15402>
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