Re: [GHC] #10587: Suspending and unsuspending ghci kills and spawns threads

29 Jun 2015

      #10587: Suspending and unsuspending ghci kills and spawns threads
---------------------------------+-----------------------------------------
        Reporter:  niteria       |                   Owner:  niteria
            Type:  bug           |                  Status:  new
        Priority:  low           |               Milestone:
       Component:  GHCi          |                 Version:  7.10.1
      Resolution:                |                Keywords:
Operating System:  Linux         |            Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |               Test Case:
      Blocked By:                |                Blocking:
 Related Tickets:                |  Differential Revisions:
---------------------------------+-----------------------------------------
Description changed by niteria:

Old description:
...
When you run:
{{{
ghci -j8
# in a different terminal:
pidstat -t -p $(pidof ghc) | grep ghc_worker
kill -SIGSTOP $(pidof ghc); kill -SIGCONT $(pidof ghc)
# SIGCONT doesn't really resume it, you have to run fg in the terminal
where it runs
pidstat -t -p $(pidof ghc) | grep ghc_worker
kill -SIGSTOP $(pidof ghc); kill -SIGCONT $(pidof ghc)
pidstat -t -p $(pidof ghc) | grep ghc_worker
kill -SIGSTOP $(pidof ghc); kill -SIGCONT $(pidof ghc)
pidstat -t -p $(pidof ghc) | grep ghc_worker
}}}
You get:
{{{
$ pidstat -t -p $(pidof ghc) | grep ghc_worker
06:40:55 AM         -   2848953    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:40:55 AM         -   2848954    0.00    0.00    0.00    0.00    31
|__ghc_worker
06:40:55 AM         -   2848955    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:40:55 AM         -   2848957    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:40:55 AM         -   2848958    0.00    0.00    0.00    0.00    22
|__ghc_worker
06:40:55 AM         -   2848959    0.00    0.00    0.00    0.00    37
|__ghc_worker
06:40:55 AM         -   2848960    0.00    0.00    0.00    0.00     3
|__ghc_worker
06:40:55 AM         -   2848961    0.00    0.00    0.00    0.00     2
|__ghc_worker
06:40:55 AM         -   2848962    0.00    0.00    0.00    0.00    36
|__ghc_worker
06:40:55 AM         -   2848963    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:40:55 AM         -   2848964    0.00    0.00    0.00    0.00    31
|__ghc_worker
06:40:55 AM         -   2848965    0.00    0.00    0.00    0.00    11
|__ghc_worker
06:40:55 AM         -   2848966    0.00    0.00    0.00    0.00    24
|__ghc_worker
06:40:55 AM         -   2848967    0.00    0.00    0.00    0.00    38
|__ghc_worker
06:40:55 AM         -   2848968    0.00    0.00    0.00    0.00    23
|__ghc_worker
06:40:55 AM         -   2848969    0.00    0.00    0.00    0.00    22
|__ghc_worker
06:40:55 AM         -   2848970    0.00    0.00    0.00    0.00    31
|__ghc_worker
}}}
{{{
$ pidstat -t -p $(pidof ghc) | grep ghc_worker
06:41:37 AM         -   2848953    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:41:37 AM         -   2848954    0.00    0.00    0.00    0.00    31
|__ghc_worker
06:41:37 AM         -   2848955    0.00    0.00    0.00    0.00     3
|__ghc_worker
06:41:37 AM         -   2848957    0.00    0.00    0.00    0.00     3
|__ghc_worker
06:41:37 AM         -   2848958    0.00    0.00    0.00    0.00    22
|__ghc_worker
06:41:37 AM         -   2848959    0.00    0.00    0.00    0.00    37
|__ghc_worker
06:41:37 AM         -   2848960    0.00    0.00    0.00    0.00     5
|__ghc_worker
06:41:37 AM         -   2848961    0.00    0.00    0.00    0.00     5
|__ghc_worker
06:41:37 AM         -   2848962    0.00    0.00    0.00    0.00    36
|__ghc_worker
06:41:37 AM         -   2848963    0.00    0.00    0.00    0.00     7
|__ghc_worker
06:41:37 AM         -   2848964    0.00    0.00    0.00    0.00    12
|__ghc_worker
06:41:37 AM         -   2848965    0.00    0.00    0.00    0.00    11
|__ghc_worker
06:41:37 AM         -   2848966    0.00    0.00    0.00    0.00    24
|__ghc_worker
06:41:37 AM         -   2848967    0.00    0.00    0.00    0.00    38
|__ghc_worker
06:41:37 AM         -   2848968    0.00    0.00    0.00    0.00    23
|__ghc_worker
06:41:37 AM         -   2848969    0.00    0.00    0.00    0.00     8
|__ghc_worker
06:41:37 AM         -   2848970    0.00    0.00    0.00    0.00    14
|__ghc_worker
06:41:37 AM         -   2850193    0.00    0.00    0.00    0.00    32
|__ghc_worker
06:41:37 AM         -   2850194    0.00    0.00    0.00    0.00    33
|__ghc_worker
06:41:37 AM         -   2850196    0.00    0.00    0.00    0.00    34
|__ghc_worker
06:41:37 AM         -   2850197    0.00    0.00    0.00    0.00    35
|__ghc_worker
06:41:37 AM         -   2850198    0.00    0.00    0.00    0.00    10
|__ghc_worker
06:41:37 AM         -   2850199    0.00    0.00    0.00    0.00    17
|__ghc_worker
06:41:37 AM         -   2850294    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:41:37 AM         -   2850295    0.00    0.00    0.00    0.00    31
|__ghc_worker
}}}
{{{
$ pidstat -t -p $(pidof ghc) | grep ghc_worker
06:42:43 AM         -   2848953    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:42:43 AM         -   2848954    0.00    0.00    0.00    0.00    31
|__ghc_worker
06:42:43 AM         -   2848955    0.00    0.00    0.00    0.00     3
|__ghc_worker
06:42:43 AM         -   2848957    0.00    0.00    0.00    0.00     7
|__ghc_worker
06:42:43 AM         -   2848958    0.00    0.00    0.00    0.00    22
|__ghc_worker
06:42:43 AM         -   2848959    0.00    0.00    0.00    0.00    37
|__ghc_worker
06:42:43 AM         -   2848960    0.00    0.00    0.00    0.00     9
|__ghc_worker
06:42:43 AM         -   2848961    0.00    0.00    0.00    0.00     0
|__ghc_worker
06:42:43 AM         -   2848962    0.00    0.00    0.00    0.00    36
|__ghc_worker
06:42:43 AM         -   2848963    0.00    0.00    0.00    0.00     4
|__ghc_worker
06:42:43 AM         -   2848964    0.00    0.00    0.00    0.00    12
|__ghc_worker
06:42:43 AM         -   2848965    0.00    0.00    0.00    0.00    11
|__ghc_worker
06:42:43 AM         -   2848966    0.00    0.00    0.00    0.00    24
|__ghc_worker
06:42:43 AM         -   2848967    0.00    0.00    0.00    0.00    38
|__ghc_worker
06:42:43 AM         -   2848968    0.00    0.00    0.00    0.00    23
|__ghc_worker
06:42:43 AM         -   2848969    0.00    0.00    0.00    0.00    29
|__ghc_worker
06:42:43 AM         -   2848970    0.00    0.00    0.00    0.00    14
|__ghc_worker
06:42:43 AM         -   2850193    0.00    0.00    0.00    0.00    32
|__ghc_worker
06:42:43 AM         -   2850196    0.00    0.00    0.00    0.00    34
|__ghc_worker
06:42:43 AM         -   2850197    0.00    0.00    0.00    0.00    35
|__ghc_worker
06:42:43 AM         -   2850198    0.00    0.00    0.00    0.00    10
|__ghc_worker
06:42:43 AM         -   2850199    0.00    0.00    0.00    0.00    19
|__ghc_worker
06:42:43 AM         -   2850295    0.00    0.00    0.00    0.00    13
|__ghc_worker
06:42:43 AM         -   2861009    0.00    0.00    0.00    0.00    34
|__ghc_worker
06:42:43 AM         -   2861010    0.00    0.00    0.00    0.00    35
|__ghc_worker
}}}

...
{{{
$ pidstat -t -p $(pidof ghc) | grep ghc_worker
06:43:37 AM         -   2848953    0.00    0.00    0.00    0.00    21
|__ghc_worker
06:43:37 AM         -   2848954    0.00    0.00    0.00    0.00    31
|__ghc_worker
06:43:37 AM         -   2848955    0.00    0.00    0.00    0.00     9
|__ghc_worker
06:43:37 AM         -   2848957    0.00    0.00    0.00    0.00     9
|__ghc_worker
06:43:37 AM         -   2848958    0.00    0.00    0.00    0.00    22
|__ghc_worker
06:43:37 AM         -   2848959    0.00    0.00    0.00    0.00    37
|__ghc_worker
06:43:37 AM         -   2848960    0.00    0.00    0.00    0.00    29
|__ghc_worker
06:43:37 AM         -   2848961    0.00    0.00    0.00    0.00    20
|__ghc_worker
06:43:37 AM         -   2848962    0.00    0.00    0.00    0.00    36
|__ghc_worker
06:43:37 AM         -   2848963    0.00    0.00    0.00    0.00     3
|__ghc_worker
06:43:37 AM         -   2848964    0.00    0.00    0.00    0.00    12
|__ghc_worker
06:43:37 AM         -   2848965    0.00    0.00    0.00    0.00    15
|__ghc_worker
06:43:37 AM         -   2848966    0.00    0.00    0.00    0.00    24
|__ghc_worker
06:43:37 AM         -   2848967    0.00    0.00    0.00    0.00    38
|__ghc_worker
06:43:37 AM         -   2848968    0.00    0.00    0.00    0.00    23
|__ghc_worker
06:43:37 AM         -   2848969    0.00    0.00    0.00    0.00     1
|__ghc_worker
06:43:37 AM         -   2848970    0.00    0.00    0.00    0.00    19
|__ghc_worker
06:43:37 AM         -   2850193    0.00    0.00    0.00    0.00    32
|__ghc_worker
06:43:37 AM         -   2850196    0.00    0.00    0.00    0.00    34
|__ghc_worker
06:43:37 AM         -   2850197    0.00    0.00    0.00    0.00    35
|__ghc_worker
06:43:37 AM         -   2850198    0.00    0.00    0.00    0.00    30
|__ghc_worker
06:43:37 AM         -   2861009    0.00    0.00    0.00    0.00    17
|__ghc_worker
06:43:37 AM         -   2861010    0.00    0.00    0.00    0.00    19
|__ghc_worker
06:43:37 AM         -   2862954    0.00    0.00    0.00    0.00     2
|__ghc_worker
06:43:37 AM         -   2862956    0.00    0.00    0.00    0.00    26
|__ghc_worker
}}}

...
The sets of threads are changing on every suspend, unsuspend combination.
This is really visible when running in gdb with -j40, gdb just spews
threads getting spawned and killed:
{{{
[Thread 0x7f0b767fc700 (LWP 2881933) exited]
[New Thread 0x7f0b767fc700 (LWP 2881940)]
[New Thread 0x7f0cc2cfd700 (LWP 2881941)]
[Thread 0x7f0b76ffd700 (LWP 2881928) exited]
[New Thread 0x7f0b7dffb700 (LWP 2881942)]
[New Thread 0x7f0b76ffd700 (LWP 2881943)]
[New Thread 0x7f0b7ffff700 (LWP 2881944)]
[Thread 0x7f0b777fe700 (LWP 2881932) exited]
[Thread 0x7f0b77fff700 (LWP 2881931) exited]
[Thread 0x7f0b7d7fa700 (LWP 2881927) exited]
[Thread 0x7f0b7f7fe700 (LWP 2881926) exited]
[New Thread 0x7f0b7f7fe700 (LWP 2881945)]
[New Thread 0x7f0b7d7fa700 (LWP 2881948)]
[Thread 0x7f0cc2cfd700 (LWP 2881941) exited]
[Thread 0x7f0b75ffb700 (LWP 2881938) exited]
[New Thread 0x7f0cc2cfd700 (LWP 2881949)]
[Thread 0x7f0b767fc700 (LWP 2881940) exited]
[New Thread 0x7f0b767fc700 (LWP 2881950)]
[New Thread 0x7f0b75ffb700 (LWP 2881951)]
[New Thread 0x7f0b77fff700 (LWP 2881952)]
[Thread 0x7f0b757fa700 (LWP 2881939) exited]
[Thread 0x7f0b7d7fa700 (LWP 2881948) exited]
[New Thread 0x7f0b757fa700 (LWP 2881953)]
[Thread 0x7f0b7effd700 (LWP 2881936) exited]
[Thread 0x7f0cc3cff700 (LWP 2881937) exited]
[New Thread 0x7f0cc3cff700 (LWP 2881954)]
[New Thread 0x7f0b7effd700 (LWP 2881955)]
[New Thread 0x7f0b7d7fa700 (LWP 2881956)]
[Thread 0x7f0b7dffb700 (LWP 2881942) exited]
[New Thread 0x7f0b777fe700 (LWP 2881957)]
[New Thread 0x7f0b7dffb700 (LWP 2881958)]
[Thread 0x7f0b76ffd700 (LWP 2881943) exited]
[New Thread 0x7f0b76ffd700 (LWP 2881959)]
[Thread 0x7f0b7e7fc700 (LWP 2881935) exited]
[Thread 0x7f0b7effd700 (LWP 2881955) exited]
[New Thread 0x7f0b7effd700 (LWP 2881962)]
[New Thread 0x7f0b7e7fc700 (LWP 2881963)]
[Thread 0x7f0b7f7fe700 (LWP 2881945) exited]
[New Thread 0x7f0b7f7fe700 (LWP 2881964)]
[New Thread 0x7f0b7cff9700 (LWP 2881965)]
[Thread 0x7f0b75ffb700 (LWP 2881951) exited]
[Thread 0x7f0b7dffb700 (LWP 2881958) exited]
[Thread 0x7f0cc3cff700 (LWP 2881954) exited]
[New Thread 0x7f0cc3cff700 (LWP 2881967)]
[New Thread 0x7f0b7dffb700 (LWP 2881968)]
[Thread 0x7f0b777fe700 (LWP 2881957) exited]
[Thread 0x7f0b757fa700 (LWP 2881953) exited]
[Thread 0x7f0cc2cfd700 (LWP 2881949) exited]
[Thread 0x7f0b7ffff700 (LWP 2881944) exited]
[New Thread 0x7f0cc2cfd700 (LWP 2881969)]
[Thread 0x7f0b7e7fc700 (LWP 2881963) exited]
[Thread 0x7f0b767fc700 (LWP 2881950) exited]
[New Thread 0x7f0b767fc700 (LWP 2881970)]
[New Thread 0x7f0b7e7fc700 (LWP 2881971)]
[Thread 0x7f0b77fff700 (LWP 2881952) exited]
[New Thread 0x7f0b7ffff700 (LWP 2881972)]
[New Thread 0x7f0b77fff700 (LWP 2881973)]
[New Thread 0x7f0b757fa700 (LWP 2881974)]
[Thread 0x7f0b7dffb700 (LWP 2881968) exited]
[New Thread 0x7f0b7dffb700 (LWP 2881975)]
[Thread 0x7f0cc2cfd700 (LWP 2881969) exited]
[New Thread 0x7f0cc2cfd700 (LWP 2881976)]
[Thread 0x7f0b76ffd700 (LWP 2881959) exited]
[Thread 0x7f0b757fa700 (LWP 2881974) exited]
[New Thread 0x7f0b76ffd700 (LWP 2881977)]
[New Thread 0x7f0b757fa700 (LWP 2881978)]
[New Thread 0x7f0b777fe700 (LWP 2881979)]
[Thread 0x7f0b7effd700 (LWP 2881962) exited]
[New Thread 0x7f0b7effd700 (LWP 2881980)]
[Thread 0x7f0b7ffff700 (LWP 2881972) exited]
[Thread 0x7f0b7e7fc700 (LWP 2881971) exited]
[Thread 0x7f0b77fff700 (LWP 2881973) exited]
[New Thread 0x7f0b77fff700 (LWP 2881981)]
[Thread 0x7f0b7d7fa700 (LWP 2881956) exited]
[New Thread 0x7f0b7d7fa700 (LWP 2881982)]
[New Thread 0x7f0b7e7fc700 (LWP 2881983)]
[Thread 0x7f0cc2cfd700 (LWP 2881976) exited]
[New Thread 0x7f0cc2cfd700 (LWP 2881984)]
[New Thread 0x7f0b7ffff700 (LWP 2881985)]
[New Thread 0x7f0b75ffb700 (LWP 2881986)]
[Thread 0x7f0b7dffb700 (LWP 2881975) exited]
[New Thread 0x7f0b7dffb700 (LWP 2881987)]
[New Thread 0x7f0b74ff9700 (LWP 2881988)]
[Thread 0x7f0b777fe700 (LWP 2881979) exited]
[Thread 0x7f0b7effd700 (LWP 2881980) exited]
[Thread 0x7f0b76ffd700 (LWP 2881977) exited]
}}}
New description:

 When you run:

 {{{
 ghci -j8
 ghci> :set -package array

 # in a different terminal:
 pidstat -t -p $(pidof ghc) | grep ghc_worker
 kill -SIGSTOP $(pidof ghc); kill -SIGCONT $(pidof ghc)
 # SIGCONT doesn't really resume it, you have to run fg in the terminal
 where it runs
 pidstat -t -p $(pidof ghc) | grep ghc_worker
 kill -SIGSTOP $(pidof ghc); kill -SIGCONT $(pidof ghc)
 pidstat -t -p $(pidof ghc) | grep ghc_worker
 kill -SIGSTOP $(pidof ghc); kill -SIGCONT $(pidof ghc)
 pidstat -t -p $(pidof ghc) | grep ghc_worker
 }}}

 You get:

 {{{
 $ pidstat -t -p $(pidof ghc) | grep ghc_worker
 06:40:55 AM         -   2848953    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:40:55 AM         -   2848954    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 06:40:55 AM         -   2848955    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:40:55 AM         -   2848957    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:40:55 AM         -   2848958    0.00    0.00    0.00    0.00    22
 |__ghc_worker
 06:40:55 AM         -   2848959    0.00    0.00    0.00    0.00    37
 |__ghc_worker
 06:40:55 AM         -   2848960    0.00    0.00    0.00    0.00     3
 |__ghc_worker
 06:40:55 AM         -   2848961    0.00    0.00    0.00    0.00     2
 |__ghc_worker
 06:40:55 AM         -   2848962    0.00    0.00    0.00    0.00    36
 |__ghc_worker
 06:40:55 AM         -   2848963    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:40:55 AM         -   2848964    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 06:40:55 AM         -   2848965    0.00    0.00    0.00    0.00    11
 |__ghc_worker
 06:40:55 AM         -   2848966    0.00    0.00    0.00    0.00    24
 |__ghc_worker
 06:40:55 AM         -   2848967    0.00    0.00    0.00    0.00    38
 |__ghc_worker
 06:40:55 AM         -   2848968    0.00    0.00    0.00    0.00    23
 |__ghc_worker
 06:40:55 AM         -   2848969    0.00    0.00    0.00    0.00    22
 |__ghc_worker
 06:40:55 AM         -   2848970    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 }}}

 {{{
 $ pidstat -t -p $(pidof ghc) | grep ghc_worker
 06:41:37 AM         -   2848953    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:41:37 AM         -   2848954    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 06:41:37 AM         -   2848955    0.00    0.00    0.00    0.00     3
 |__ghc_worker
 06:41:37 AM         -   2848957    0.00    0.00    0.00    0.00     3
 |__ghc_worker
 06:41:37 AM         -   2848958    0.00    0.00    0.00    0.00    22
 |__ghc_worker
 06:41:37 AM         -   2848959    0.00    0.00    0.00    0.00    37
 |__ghc_worker
 06:41:37 AM         -   2848960    0.00    0.00    0.00    0.00     5
 |__ghc_worker
 06:41:37 AM         -   2848961    0.00    0.00    0.00    0.00     5
 |__ghc_worker
 06:41:37 AM         -   2848962    0.00    0.00    0.00    0.00    36
 |__ghc_worker
 06:41:37 AM         -   2848963    0.00    0.00    0.00    0.00     7
 |__ghc_worker
 06:41:37 AM         -   2848964    0.00    0.00    0.00    0.00    12
 |__ghc_worker
 06:41:37 AM         -   2848965    0.00    0.00    0.00    0.00    11
 |__ghc_worker
 06:41:37 AM         -   2848966    0.00    0.00    0.00    0.00    24
 |__ghc_worker
 06:41:37 AM         -   2848967    0.00    0.00    0.00    0.00    38
 |__ghc_worker
 06:41:37 AM         -   2848968    0.00    0.00    0.00    0.00    23
 |__ghc_worker
 06:41:37 AM         -   2848969    0.00    0.00    0.00    0.00     8
 |__ghc_worker
 06:41:37 AM         -   2848970    0.00    0.00    0.00    0.00    14
 |__ghc_worker
 06:41:37 AM         -   2850193    0.00    0.00    0.00    0.00    32
 |__ghc_worker
 06:41:37 AM         -   2850194    0.00    0.00    0.00    0.00    33
 |__ghc_worker
 06:41:37 AM         -   2850196    0.00    0.00    0.00    0.00    34
 |__ghc_worker
 06:41:37 AM         -   2850197    0.00    0.00    0.00    0.00    35
 |__ghc_worker
 06:41:37 AM         -   2850198    0.00    0.00    0.00    0.00    10
 |__ghc_worker
 06:41:37 AM         -   2850199    0.00    0.00    0.00    0.00    17
 |__ghc_worker
 06:41:37 AM         -   2850294    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:41:37 AM         -   2850295    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 }}}

 {{{
 $ pidstat -t -p $(pidof ghc) | grep ghc_worker
 06:42:43 AM         -   2848953    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:42:43 AM         -   2848954    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 06:42:43 AM         -   2848955    0.00    0.00    0.00    0.00     3
 |__ghc_worker
 06:42:43 AM         -   2848957    0.00    0.00    0.00    0.00     7
 |__ghc_worker
 06:42:43 AM         -   2848958    0.00    0.00    0.00    0.00    22
 |__ghc_worker
 06:42:43 AM         -   2848959    0.00    0.00    0.00    0.00    37
 |__ghc_worker
 06:42:43 AM         -   2848960    0.00    0.00    0.00    0.00     9
 |__ghc_worker
 06:42:43 AM         -   2848961    0.00    0.00    0.00    0.00     0
 |__ghc_worker
 06:42:43 AM         -   2848962    0.00    0.00    0.00    0.00    36
 |__ghc_worker
 06:42:43 AM         -   2848963    0.00    0.00    0.00    0.00     4
 |__ghc_worker
 06:42:43 AM         -   2848964    0.00    0.00    0.00    0.00    12
 |__ghc_worker
 06:42:43 AM         -   2848965    0.00    0.00    0.00    0.00    11
 |__ghc_worker
 06:42:43 AM         -   2848966    0.00    0.00    0.00    0.00    24
 |__ghc_worker
 06:42:43 AM         -   2848967    0.00    0.00    0.00    0.00    38
 |__ghc_worker
 06:42:43 AM         -   2848968    0.00    0.00    0.00    0.00    23
 |__ghc_worker
 06:42:43 AM         -   2848969    0.00    0.00    0.00    0.00    29
 |__ghc_worker
 06:42:43 AM         -   2848970    0.00    0.00    0.00    0.00    14
 |__ghc_worker
 06:42:43 AM         -   2850193    0.00    0.00    0.00    0.00    32
 |__ghc_worker
 06:42:43 AM         -   2850196    0.00    0.00    0.00    0.00    34
 |__ghc_worker
 06:42:43 AM         -   2850197    0.00    0.00    0.00    0.00    35
 |__ghc_worker
 06:42:43 AM         -   2850198    0.00    0.00    0.00    0.00    10
 |__ghc_worker
 06:42:43 AM         -   2850199    0.00    0.00    0.00    0.00    19
 |__ghc_worker
 06:42:43 AM         -   2850295    0.00    0.00    0.00    0.00    13
 |__ghc_worker
 06:42:43 AM         -   2861009    0.00    0.00    0.00    0.00    34
 |__ghc_worker
 06:42:43 AM         -   2861010    0.00    0.00    0.00    0.00    35
 |__ghc_worker
 }}}

 {{{
 $ pidstat -t -p $(pidof ghc) | grep ghc_worker
 06:43:37 AM         -   2848953    0.00    0.00    0.00    0.00    21
 |__ghc_worker
 06:43:37 AM         -   2848954    0.00    0.00    0.00    0.00    31
 |__ghc_worker
 06:43:37 AM         -   2848955    0.00    0.00    0.00    0.00     9
 |__ghc_worker
 06:43:37 AM         -   2848957    0.00    0.00    0.00    0.00     9
 |__ghc_worker
 06:43:37 AM         -   2848958    0.00    0.00    0.00    0.00    22
 |__ghc_worker
 06:43:37 AM         -   2848959    0.00    0.00    0.00    0.00    37
 |__ghc_worker
 06:43:37 AM         -   2848960    0.00    0.00    0.00    0.00    29
 |__ghc_worker
 06:43:37 AM         -   2848961    0.00    0.00    0.00    0.00    20
 |__ghc_worker
 06:43:37 AM         -   2848962    0.00    0.00    0.00    0.00    36
 |__ghc_worker
 06:43:37 AM         -   2848963    0.00    0.00    0.00    0.00     3
 |__ghc_worker
 06:43:37 AM         -   2848964    0.00    0.00    0.00    0.00    12
 |__ghc_worker
 06:43:37 AM         -   2848965    0.00    0.00    0.00    0.00    15
 |__ghc_worker
 06:43:37 AM         -   2848966    0.00    0.00    0.00    0.00    24
 |__ghc_worker
 06:43:37 AM         -   2848967    0.00    0.00    0.00    0.00    38
 |__ghc_worker
 06:43:37 AM         -   2848968    0.00    0.00    0.00    0.00    23
 |__ghc_worker
 06:43:37 AM         -   2848969    0.00    0.00    0.00    0.00     1
 |__ghc_worker
 06:43:37 AM         -   2848970    0.00    0.00    0.00    0.00    19
 |__ghc_worker
 06:43:37 AM         -   2850193    0.00    0.00    0.00    0.00    32
 |__ghc_worker
 06:43:37 AM         -   2850196    0.00    0.00    0.00    0.00    34
 |__ghc_worker
 06:43:37 AM         -   2850197    0.00    0.00    0.00    0.00    35
 |__ghc_worker
 06:43:37 AM         -   2850198    0.00    0.00    0.00    0.00    30
 |__ghc_worker
 06:43:37 AM         -   2861009    0.00    0.00    0.00    0.00    17
 |__ghc_worker
 06:43:37 AM         -   2861010    0.00    0.00    0.00    0.00    19
 |__ghc_worker
 06:43:37 AM         -   2862954    0.00    0.00    0.00    0.00     2
 |__ghc_worker
 06:43:37 AM         -   2862956    0.00    0.00    0.00    0.00    26
 |__ghc_worker
 }}}

 The sets of threads are changing on every suspend, unsuspend combination.

 This is really visible when running in gdb with -j40, gdb just spews
 threads getting spawned and killed:
 {{{
 [Thread 0x7f0b767fc700 (LWP 2881933) exited]
 [New Thread 0x7f0b767fc700 (LWP 2881940)]
 [New Thread 0x7f0cc2cfd700 (LWP 2881941)]
 [Thread 0x7f0b76ffd700 (LWP 2881928) exited]
 [New Thread 0x7f0b7dffb700 (LWP 2881942)]
 [New Thread 0x7f0b76ffd700 (LWP 2881943)]
 [New Thread 0x7f0b7ffff700 (LWP 2881944)]
 [Thread 0x7f0b777fe700 (LWP 2881932) exited]
 [Thread 0x7f0b77fff700 (LWP 2881931) exited]
 [Thread 0x7f0b7d7fa700 (LWP 2881927) exited]
 [Thread 0x7f0b7f7fe700 (LWP 2881926) exited]
 [New Thread 0x7f0b7f7fe700 (LWP 2881945)]
 [New Thread 0x7f0b7d7fa700 (LWP 2881948)]
 [Thread 0x7f0cc2cfd700 (LWP 2881941) exited]
 [Thread 0x7f0b75ffb700 (LWP 2881938) exited]
 [New Thread 0x7f0cc2cfd700 (LWP 2881949)]
 [Thread 0x7f0b767fc700 (LWP 2881940) exited]
 [New Thread 0x7f0b767fc700 (LWP 2881950)]
 [New Thread 0x7f0b75ffb700 (LWP 2881951)]
 [New Thread 0x7f0b77fff700 (LWP 2881952)]
 [Thread 0x7f0b757fa700 (LWP 2881939) exited]
 [Thread 0x7f0b7d7fa700 (LWP 2881948) exited]
 [New Thread 0x7f0b757fa700 (LWP 2881953)]
 [Thread 0x7f0b7effd700 (LWP 2881936) exited]
 [Thread 0x7f0cc3cff700 (LWP 2881937) exited]
 [New Thread 0x7f0cc3cff700 (LWP 2881954)]
 [New Thread 0x7f0b7effd700 (LWP 2881955)]
 [New Thread 0x7f0b7d7fa700 (LWP 2881956)]
 [Thread 0x7f0b7dffb700 (LWP 2881942) exited]
 [New Thread 0x7f0b777fe700 (LWP 2881957)]
 [New Thread 0x7f0b7dffb700 (LWP 2881958)]
 [Thread 0x7f0b76ffd700 (LWP 2881943) exited]
 [New Thread 0x7f0b76ffd700 (LWP 2881959)]
 [Thread 0x7f0b7e7fc700 (LWP 2881935) exited]
 [Thread 0x7f0b7effd700 (LWP 2881955) exited]
 [New Thread 0x7f0b7effd700 (LWP 2881962)]
 [New Thread 0x7f0b7e7fc700 (LWP 2881963)]
 [Thread 0x7f0b7f7fe700 (LWP 2881945) exited]
 [New Thread 0x7f0b7f7fe700 (LWP 2881964)]
 [New Thread 0x7f0b7cff9700 (LWP 2881965)]
 [Thread 0x7f0b75ffb700 (LWP 2881951) exited]
 [Thread 0x7f0b7dffb700 (LWP 2881958) exited]
 [Thread 0x7f0cc3cff700 (LWP 2881954) exited]
 [New Thread 0x7f0cc3cff700 (LWP 2881967)]
 [New Thread 0x7f0b7dffb700 (LWP 2881968)]
 [Thread 0x7f0b777fe700 (LWP 2881957) exited]
 [Thread 0x7f0b757fa700 (LWP 2881953) exited]
 [Thread 0x7f0cc2cfd700 (LWP 2881949) exited]
 [Thread 0x7f0b7ffff700 (LWP 2881944) exited]
 [New Thread 0x7f0cc2cfd700 (LWP 2881969)]
 [Thread 0x7f0b7e7fc700 (LWP 2881963) exited]
 [Thread 0x7f0b767fc700 (LWP 2881950) exited]
 [New Thread 0x7f0b767fc700 (LWP 2881970)]
 [New Thread 0x7f0b7e7fc700 (LWP 2881971)]
 [Thread 0x7f0b77fff700 (LWP 2881952) exited]
 [New Thread 0x7f0b7ffff700 (LWP 2881972)]
 [New Thread 0x7f0b77fff700 (LWP 2881973)]
 [New Thread 0x7f0b757fa700 (LWP 2881974)]
 [Thread 0x7f0b7dffb700 (LWP 2881968) exited]
 [New Thread 0x7f0b7dffb700 (LWP 2881975)]
 [Thread 0x7f0cc2cfd700 (LWP 2881969) exited]
 [New Thread 0x7f0cc2cfd700 (LWP 2881976)]
 [Thread 0x7f0b76ffd700 (LWP 2881959) exited]
 [Thread 0x7f0b757fa700 (LWP 2881974) exited]
 [New Thread 0x7f0b76ffd700 (LWP 2881977)]
 [New Thread 0x7f0b757fa700 (LWP 2881978)]
 [New Thread 0x7f0b777fe700 (LWP 2881979)]
 [Thread 0x7f0b7effd700 (LWP 2881962) exited]
 [New Thread 0x7f0b7effd700 (LWP 2881980)]
 [Thread 0x7f0b7ffff700 (LWP 2881972) exited]
 [Thread 0x7f0b7e7fc700 (LWP 2881971) exited]
 [Thread 0x7f0b77fff700 (LWP 2881973) exited]
 [New Thread 0x7f0b77fff700 (LWP 2881981)]
 [Thread 0x7f0b7d7fa700 (LWP 2881956) exited]
 [New Thread 0x7f0b7d7fa700 (LWP 2881982)]
 [New Thread 0x7f0b7e7fc700 (LWP 2881983)]
 [Thread 0x7f0cc2cfd700 (LWP 2881976) exited]
 [New Thread 0x7f0cc2cfd700 (LWP 2881984)]
 [New Thread 0x7f0b7ffff700 (LWP 2881985)]
 [New Thread 0x7f0b75ffb700 (LWP 2881986)]
 [Thread 0x7f0b7dffb700 (LWP 2881975) exited]
 [New Thread 0x7f0b7dffb700 (LWP 2881987)]
 [New Thread 0x7f0b74ff9700 (LWP 2881988)]
 [Thread 0x7f0b777fe700 (LWP 2881979) exited]
 [Thread 0x7f0b7effd700 (LWP 2881980) exited]
 [Thread 0x7f0b76ffd700 (LWP 2881977) exited]
 }}}

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10587#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler