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Simple loop

This HOWTO describes how to dump a trivial program.

Contents

Simplest caseEdit

PrepareEdit

Let's create a shell script which loops and reports time every second:

$ cat > test.sh <<-EOF
#!/bin/sh
while :; do
    sleep 1
    date
done
EOF
$ chmod +x test.sh

Run (isolated)Edit

Now, if you try to simply run and try to dump such a program, criu will fail. The reason is a program you launch from the shell shares some common resources with the shell, notably its session and terminal. Criu has a built-in check that makes sure there are no such resources.

To remove the dependency on a current terminal, let's executed our script in a new session and redirect its output to a file:

$ setsid ./test.sh  < /dev/null &> test.log &
[2] 2220
[2]+  Done                    setsid ./test.sh < /dev/null &>test.log

DumpEdit

Get the PID of the test process:

$  ps -C test.sh
 PID TTY          TIME CMD
2221 ?        00:00:00 test.sh

Dump it:

# criu dump -t 2221 -vvv -o dump.log && echo OK
OK

Tip: Alternatively, you can combine both steps into a single command: # criu dump -t $(pgrep test.sh) -v4 -o dump.log

Check dump filesEdit

The state of the process(es) is saved to a few files:

$  ls
core-2221.img   eventpoll-tfd.img  filelocks-2221.img  inotify.img       netlinksk.img     pipes.img         sigacts-2424.img   test.log
core-2424.img   fanotify.img       filelocks-2424.img  inotify-wd.img    packetsk.img      pstree.img        signalfd.img       test.sh
creds-2221.img  fanotify-mark.img  fs-2221.img         inventory.img     pagemap-2221.img  reg-files.img     signal-p-2221.img  tty.img
creds-2424.img  fdinfo-2.img       fs-2424.img         itimers-2221.img  pagemap-2424.img  remap-fpath.img   signal-p-2424.img  tty-info.img
dump.log        fdinfo-3.img       ids-2221.img        itimers-2424.img  pages-1.img       rlimit-2221.img   signal-s-2221.img  unixsk.img
eventfd.img     fifo-data.img      ids-2424.img        mm-2221.img       pages-2.img       rlimit-2424.img   signal-s-2424.img  vmas-2221.img
eventpoll.img   fifo.img           inetsk.img          mm-2424.img       pipes-data.img    sigacts-2221.img  sk-queues.img      vmas-2424.img

RestoreEdit

Restore the test process:

# criu restore -d -vvv -o restore.log && echo OK
OK

A shell jobEdit

Now let's see how to dump the same program when it is just started from the shell without any additional setsid and stdio redirection. For that we need to execute our test script or another program (e.g. "top") from a terminal without any redirection.

Note criu only supports Unix98 PTYs.

RunEdit

$ ./test.sh
$ ps -C test.sh
  PID TTY          TIME CMD
 2621 pts/1    00:00:00 test.sh

DumpEdit

Currently criu doesn't support a stopped task, so "criu dump" must be executed from another terminal.

# criu dump -t 2621
(00.012929) Error (tty.c:1022): tty: Found dangling tty with sid 940 pgid 2621 (pts) on peer fd 0. Consider using --shell-job option.
(00.013111) Error (cr-dump.c:1636): Dumping FAILED.

See, criu asks to set the --shell-job, because we try to dump only a part of a session and a slave end of a tty pair. It is a common rule to ask a user, if only a part of something is dumped.

# criu dump -vvvv -o dump.log -t 2621 --shell-job && echo OK
OK

RestoreEdit

#  criu restore -vvvv -o restore.log --shell-job
Fri Apr 12 12:41:09 MSK 2013
Fri Apr 12 12:41:10 MSK 2013