Perl threads- can they be named uniquely? (Linux) - linux

Question

in reference to this from perlvar:
In multithreaded scripts Perl coordinates the threads so that any thread may modify its copy of the $0 and the change becomes visible to ps(1) (assuming the operating system plays along). Note that the view of $0 the other threads have will not change since they have their own copies of it.
I don't seem to be getting this behavior. Instead, the $0 seems to be shared by all my threads, and in the ps output, the top level main perl interpreter's cmdline is being modified to the final value applied by the last thread.
eg.
My goal is instead of having this where all threads a named the same under COMMAND column:
top -b -n 1 -H -p 223860
top - 17:54:56 up 73 days, 2:15, 7 users, load average: 0.23, 0.70, 0.92
Threads: 22 total, 0 running, 22 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 32358832 total, 26418060 free, 1090028 used, 4850744 buff/cache
KiB Swap: 16777212 total, 16149116 free, 628096 used. 30804716 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
223860 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:45.75 tool_reader.
223863 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:03.88 tool_reader.
223864 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:04.67 tool_reader.
223865 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:00.00 tool_reader.
223867 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:34.62 tool_reader.
223868 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:03.85 tool_reader.
223869 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:04.41 tool_reader.
223870 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:00.00 tool_reader.
223872 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:40.14 tool_reader.
To have something more useful under the command column like this, and the main thread stays the same.
|
|
|
v
top -b -n 1 -H -p 223860
top - 17:54:56 up 73 days, 2:15, 7 users, load average: 0.23, 0.70, 0.92
Threads: 22 total, 0 running, 22 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 32358832 total, 26418060 free, 1090028 used, 4850744 buff/cache
KiB Swap: 16777212 total, 16149116 free, 628096 used. 30804716 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
223860 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:45.75 tool_reader.
223863 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:03.88 syncer
223864 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:04.67 partition1
223865 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:00.00 partition2
223867 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:34.62 partition3
223868 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:03.85 input_merger1
223869 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:04.41 input_merger2
223870 app_sy+ 20 0 3833640 166216 2500 S 0.0 0.5 0:00.00 input_merger3
Would anyone know how this can be done? I'm using a rather old perl now, version 5.16.3, in case this was a bug?

Answer 1

Update 2020-10-21:
I just discovered an even better way to achieve this - the actual linux syscall. https://man7.org/linux/man-pages/man2/prctl.2.html
Troels Liebe Bentsen has kindly contributed a module that handles this neatly.
https://metacpan.org/pod/Sys::Prctl
Far more seamless than fiddling with $0 !!!
Original Post content continues below....
ps -T -p 126193
PID SPID TTY TIME CMD
126193 126193 pts/11 00:00:00 test2.pl
126193 126194 pts/11 00:00:00 __thr1 #<--- now unique
126193 126195 pts/11 00:00:00 __thr2 #<--- now unique
top -H -p 126193
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
126193 xxxxxxx+ 20 0 305948 7972 2244 S 0.0 0.0 0:00.01 test2.pl
126194 xxxxxxx+ 20 0 305948 7972 2244 S 0.0 0.0 0:00.00 __thr1
126195 xxxxxxx+ 20 0 305948 7972 2244 S 0.0 0.0 0:00.00 __thr2
##################
Thanks to #ikegami , I found a solution that will work.
Couple of small changes were needed in order to get keep the child threads alive, and also needed to stop the main thread from joining them back in. (Based on how it behaves, I assume that if the child threads reach the end of the sub they are spawned with, they are completely terminated and Linux cleans them up - even though the main thread hasn't called join on them yet.
To anyone else reading this page in future, I would love to know why each of pstree, ps, and top, all show a different result.
Any how, leaving this info and comparisons here in case its helpful to others.
End result:
Using ps command , it does NOT appear to be possible to get the modified name of the threads. It only shows the string of what the last thread that touched $0 set it to
Similarly, using pstree pstree -p -a -l 144741 also only shows the main thread as the name for each child, and does not show anything about the changes made by the threads
But, very fortunately, using top works!!!! top -H -b -p 180547 , which clearly shows the main thread, and all child threads by the name they set using $0
Example from ps:
app_sy+ 180547 131203 180547 0 3 18:08 pts/1 00:00:00 thr2
app_sy+ 180547 131203 180548 0 3 18:08 pts/1 00:00:00 thr2
app_sy+ 180547 131203 180549 0 3 18:08 pts/1 00:00:00 thr2
Example using pstree:
test.pl,180547
|-{test.pl},180548
`-{test.pl},180549
And the winner, using top -n 1 -H -b -p 180547 , which shows the distinct names applied to $0 by each thread successfully!!!!!!
top - 18:00:08 up 69 days, 8:53, 3 users, load average: 4.10, 3.95, 4.05
Threads: 3 total, 0 running, 3 sleeping, 0 stopped, 0 zombie
%Cpu(s): 7.7 us, 33.5 sy, 0.0 ni, 58.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 13144056+total, 1351640 free, 45880316 used, 84208608 buff/cache
KiB Swap: 16777212 total, 16777212 free, 0 used. 78196224 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
180547 app_+ 20 0 299572 7152 2144 S 0.0 0.0 0:00.00 test.pl
180548 app_+ 20 0 299572 7152 2144 S 0.0 0.0 0:00.02 thr1
180549 app_+ 20 0 299572 7152 2144 S 0.0 0.0 0:00.01 thr2
Adding Modified version of Ikegami's code here for future reference for others looking at this page, saved as test.pl :
#!/usr/bin/perl
use strict;
use warnings;
use feature qw( say );
use threads;
use threads::shared;
my $phase :shared = 0;
my $main_pid = $$;
sub advance {
lock $phase;
++$phase;
cond_signal($phase);
}
sub wait_for {
lock $phase;
cond_wait($phase) while $phase != $_[0];
}
sub advance_and_wait_for {
lock $phase;
++$phase;
cond_signal($phase);
cond_wait($phase) while $phase != $_[0];
}
my $thr1 = async {
my $id = 'thr1';
wait_for(0);
advance_and_wait_for(2);
say "[$id] Setting \$0 to $id.";
$0 = $id;
say "[$id] \$0 = $0";
print `ps -eLf|grep $main_pid` =~ s/^/[$id] /mrg;
advance_and_wait_for(4);
say "[$id] \$0 = $0";
advance();
while(1){
sleep 1;
}
};
my $thr2 = async {
my $id = 'thr2';
wait_for(1);
advance_and_wait_for(3);
say "[$id] \$0 = $0";
say "[$id] Setting \$0 to $id.";
$0 = $id;
say "[$id] \$0 = $0";
print `ps -eLf| grep $main_pid` =~ s/^/[$id] /mrg;
advance();
while(1){
sleep 1;
}
};
sleep 5;
print "Main thread pid is $main_pid - and \$0 is ($0)\n";
my $waitfor = <STDIN>;
$_->join for $thr1, $thr2;