In c#, we can create thread and start thread only on need like following (if I am correct)
Thread th=new thread("function");
th.start()
But in perl, when I create itself it has started. For example
$thread1=thread->create("function");
But I want to create 4 thread. I should start only on need. And I've to check whether it's running or not? if thread is not running, then I've to start the same thread by passing different parameter. How to do that in perl ?
Multiple jobs can be sent into the queue, and they are waiting for their turn to be passed to the worker.
use strict;
use warnings;
use threads;
use Thread::Queue;
my $no_of_workers = 4;
my $q = Thread::Queue->new();
# Worker thread
my #thr = map {
my $t = threads->create(sub{
# Thread will loop until no more work
while (defined(my $item = $q->dequeue())) {
# Do work on $item
print "$item\n";
}
});
{queue => $q, thread => $t, id => $_};
} 1 .. $no_of_workers;
# Send work to each thread
$_->{queue}->enqueue("Job for thread $_->{id}") for #thr;
for (#thr) {
# Signal that there is no more work to be sent
# $_->{queue}->end();
# similar to $queue->end() for older perl
$_->{queue}->enqueue(undef) for #thr;
# wait for threads to finish
$_->{thread}->join();
}
Assigning jobs 0..19 in circular way to workers,
for my $i (0 .. 19) {
my $t = $thr[$i % #thr]; # $i % #thr => 0,1,2,3, 0,1,2,3, ..
$t->{queue}->enqueue("Job for thread $t->{id}");
}
You don't want a queue for each thread! You'll end up with idle threads even if work's available.
use strict;
use warnings;
use threads;
use Thread::Queue 3.01 qw( );
use constant NUM_WORKERS => 4;
sub worker {
my ($job) = #_;
print("Job: $job\n");
sleep(rand(4)); # Pretending to do $job
}
{
my $q = Thread::Queue->new();
for (1..NUM_WORKERS) {
async {
while (defined(my $item = $q->dequeue())) {
worker($item);
}
};
}
# Give the workers work to do.
$q->enqueue($_) for 1..14;
# When you're done adding, wait for the workers to finish.
$q->end();
$_->join() for threads->list;
}
This code only does 4 threads, and then stops. It doesn't process the remaining 6 items in the queue.
Related
I have a threaded application and would like to set timeouts for the threads. Peldoc for alarm suggests to use a eval-die pair and catch the ALRM signal. However, this fails with threads producing the error Alarm clock:
use strict; use warnings;
require threads;
require threads::shared;
my $t = threads->create( sub {
eval {
$SIG{ALRM} = sub { die "alarm\n" };
alarm 2;
main();
alarm 0;
};
if ($#){
die $# unless $# eq "alarm\n";
print "timed out\n";
}
}
);
my #r = $t->join;
print "done\n";
sub main {
sleep 3;
}
This post suggests that alarm is called without signal handler in the threads library. Another post is about this problem and answers suggest to use fork and waitpid, but I would really like to use threads. Another post claims to come up with a solution, but this still gives the Alarm clock error for me. I tried to catch Alarm clock in the if ($#), but no success. Any idea how I could make this work?
The whole idea of using alarm in threads is problematic.
Signals are sent to processes, not threads.
What if two threads want to use alarm?
You'll have to implement your own system. The following is an attempt at a general solution:
package Threads::Alarm;
use strict;
use warnings;
use threads;
use threads::shared;
use Exporter qw( import );
our #EXPORT_OK = qw( alarm thread_alarm );
# A list of "$time:$tid" strings sorted by ascending time.
my #alarms :shared;
sub thread_alarm {
my ($wait) = #_;
my $tid = threads->tid();
lock #alarms;
# Cancel existing alarm for this thread, if any.
for my $i (0..$#alarms) {
if ((split(/:/, $alarms[$i]))[1] == $tid) {
splice(#alarms, $i, 1);
last;
}
}
# Create an alarm
if ($wait) {
my $when = time() + $wait;
# A binary search would be better.
my $i;
for ($i=0; $i<#alarms; ++$i) {
last if $when < (split(/:/, $alarms[$i]))[0];
}
splice(#alarms, $i, 0, "$when:$tid");
}
# Notify others of change to #alarms.
cond_broadcast(#alarms);
}
{
no warnings 'once';
*alarm = \&thread_alarm;
}
threads->create(sub {
while (1) {
my $thread;
{
lock #alarms;
while (1) {
# Wait for an alarm request to come in.
cond_wait(#alarms) while !#alarms;
# Grab the soonest alarm.
my ($when, $tid) = split(/:/, $alarms[0]);
# Check if the thread still exists.
my $thread = threads->object($tid)
or last;
# Wait for the #alarms to change or for the alarm time.
last if !cond_timedwait(#alarms, $when);
}
# Before releasing the lock, remove the alarm we're about to raise.
shift(#alarms);
# Notify others of change to #alarms.
# Doesn't actually do anything at this time.
cond_broadcast(#alarms);
}
$thread->kill('ALRM') if $thread;
}
})->detach();
1;
Completely untested. Well, I made sure it compiles, but that's it.
Note that threads->kill doesn't send a real signal (since those are sent to processes, not threads), so the OS won't interrupt any operation (e.g. sleep, wait). Simple solution: Send a real signal to a handler that does nothing right after calling threads->kill. Maybe I should have written a solution that was based around the actual SIGALRM.
sub worker {
# Curse stuff
initscr();
cbreak();
noecho();
my $fh = FileHandle->new;
open $fh, q{-|},
"$myexe #pre_args make #after_args 2>&1"
or croak 'Cannot open';
process_output($fh);
my $err = close $fh;
endwin();
return;
}
sub process_output {
my ($fh) = #_;
while (my $line = <$fh>) {
#do stuff
}
}
ReadMode 3;
threads->create(\&worker);
while (threads->list(threads::running)) {
my $char = ReadKey -1, *STDIN;
if ($char) {
if ($char eq 'q') {
endwin();
kill('INT', $$);
threads->exit();
}
}
}
ReadMode 0;
foreach my $thr (threads->list) {
$thr->join();
When I press 'q':
Perl exited with active threads:
1 running and unjoined
0 finished and unjoined
0 running and detached
and then I did ps -fu myuserid
I saw that $myexe was still running
Q1) How can i force child process to exit? threads->exit() didnt seem to work
The most obvious problem with the sample program is that it is using multiple threads for the curses library. That won't work. (curses is not thread-safe). If you have to do this, keep all of the curses work in the same thread.
You call exit but don't detach or join the threads.
Stick:
foreach my $thr ( threads -> list() ) {
$thr -> join;
}
at the end, and your 'main' code will wait for your threads to (all) exit.
However - threads -> exit() is for exiting the current thread. See:
http://perldoc.perl.org/threads.html#EXITING-A-THREAD
In order to terminate another thread, you need something like threads -> kill. Either send a 'proper' kill signal, or use a signal handler for SIGUSR1 or similar.
I'd probably approach it a little differently though - define a shared variable:
my $done : shared;
And then test it within the while loop, so you've a normal execution flow rather than a mid flight kill.
Your kill ( INT, $$ ) is going to be killing your main process, and just drop the threads on the floor. That's not good style really.
So - to backtrack a bit - the problem you're having - I think - is because 'signals' in perl aren't what you're expecting them to be. Perl uses safe signals, which makes a blocking call (such as a read from a filehandle) block the signal.
http://perldoc.perl.org/perlipc.html#Deferred-Signals-%28Safe-Signals%29
So I wouldn't normally suggest using signals within threads are a good idea - they're a little erratic, which isn't good for program predictability.
You can 'throw' and 'catch' signals, by defining a signal handler within the thread:
$SIG{'USR1'} = sub { print "Caught USR1"; die };
And then 'call' it using:
$worker -> kill ( 'USR1' );
But in certain circumstances, that won't 'bail out' immediately in the way you expect.
For example - this will work:
#!/usr/bin/perl
use strict;
use warnings;
use threads;
sub worker {
print Dumper \%SIG;
my $tail_pid = open ( my $tail_proc, "-|", "tail -f /var/log/messages" );
$SIG{'USR1'} = sub { print "Caught USR1\nKilling $tail_pid"; kill ( 'TERM', $tail_pid ); die; threads -> exit() };
print "Pre-loop\n";
while ( 1 ) {
print "Thread processing\n";
sleep 1;
}
print "Done";
return;
}
my $worker = threads -> create ( \&worker );
sleep 2;
print "Sending kill\n";
$worker -> kill ( 'SIGUSR1' );
sleep 2;
print "waiting for join\n";
$worker -> join();
But if your while loop is reading from the file handle - it's a blocking call, so the 'kill' will be held until the block lifts.
E.g.
while ( <$tail_proc> ) {
Will go into a block pending IO, and your thread won't 'get' the signal until IO occurs, and the thread continues processing. That might be sufficient for your needs though. Otherwise you're potentially looking at select or IO::Select to test if the handle is readable.
So what you may want to do instead is just kill the process that's 'feeding' your while loop - because by doing so, it'll unblock and the while will become undef and exit.
E.g.:
#!/usr/bin/perl
use strict;
use warnings;
use threads;
use threads::shared;
my $kill_pid : shared;
sub worker {
$kill_pid = open ( my $tail_proc, "-|", "tail -f /var/log/messages" );
print "Pre-loop\n";
while ( <$tail_proc> ) {
print "Thread processing\n";
print;
sleep 1;
}
print "Done";
return;
}
my $worker = threads -> create ( \&worker );
sleep 2;
print "Sending kill\n";
if ( defined $kill_pid ) { print "killing tail, $kill_pid\n"; kill ( 'TERM', $kill_pid ); };
sleep 2;
print "waiting for join\n";
$worker -> join();
My Perl script needs to run multiple threads simultaneously...
use threads ('yield', 'exit' => 'threads_only');
use threads::shared;
use strict;
use warnings;
no warnings 'threads';
use LWP::UserAgent;
use HTTP::Request;
use HTTP::Async;
use ...
...and such threads need to obtain some information from web, so HTTP::Async is used.
my $request = HTTP::Request->new;
$request->protocol('HTTP/1.1');
$request->method('GET');
$request->header('User-Agent' => '...');
my $async = HTTP::Async->new( slots => 100,
timeout => REQUEST_TIMEOUT,
max_request_time => REQUEST_TIMEOUT );
But some threads need to access web only when other thread(s) says so.
my $start = [Time::HiRes::gettimeofday()];
my #threads = ();
foreach ... {
$thread = threads->create(
sub {
local $SIG{KILL} = sub { threads->exit };
my $url = shift;
if ($url ... ) {
# wait for "go" signal from other threads
}
my ($response, $data);
$request->url($url);
$data = '';
$async->add($request);
while ($response = $async->wait_for_next_response) {
threads->yield();
$data .= $response->as_string;
}
if ($data ... ) {
# send "go" signal to waiting threads
}
}
}, $_);
if (defined $thread) {
$thread->detach;
push (#threads, $thread);
}
}
There might be one or more threads waiting for "go" signal and there might be one or more threads that such "go" signal can send. At the beginning the status of semaphore is "wait" and once it turns to "go", it will stay so.
Finally, app checks max running time. If threads are running too long, self-termination signal is sent.
my $running;
do {
$running = 0;
foreach my $thread (#threads) {
$running++ if $thread->is_running();
}
threads->yield();
} until (($running == 0) ||
(Time::HiRes::tv_interval($start) > MAX_RUN_TIME));
$running = 0;
foreach my $thread (#threads) {
if ($thread->is_running()) {
$thread->kill('KILL');
$running++;
}
}
threads->yield();
Now to the point. My questions are:
How can I most effectively code waiting "semaphore" in the script (see comments in script above). Should I simply use just shared variable with some dummy sleep loop?
Do I need to add some sleep loop at the end of app to give time to threads for self-destruction?
You might look at Thread::Queue to perform this work. You could setup a queue that would handle the signaling between the threads waiting for the 'go' signal and the threads sending the 'go' signal. Here's a quick mock-up that I haven't tested:
...
use Thread::Queue;
...
# In main body
my $q = Thread::Queue->new();
...
$thread = threads->create(
sub {
local $SIG{KILL} = sub { threads->exit };
my $url = shift;
if ($url ... ) {
# wait for "go" signal from other threads
my $mesg = $q->dequeue();
# you could put in some termination code if the $mesg isn't 'go'
if ($mesg ne 'go') { ... }
}
...
if ($data ... ) {
# send "go" signal to waiting threads
$q->enqueue('go');
}
}
}, $_);
...
The threads that need to wait for a 'go' signal will wait on the dequeue method until something enters the queue. Once a message enters the queue one thread and only one thread will grab the message and process it.
If you wish to stop the threads so that they won't run, you can insert a stop message to the head of the queue.
$q->insert(0, 'stop') foreach (#threads);
There are examples in Thread::Queue and threads CPAN distributions that show this in more detail.
In response to your second question, the answer is, unfortunately, it depends. When you proceed to terminate your threads, what kind of clean up is required for a clean shutdown? What's the worst case scenario that could occur if the rug was yanked out from beneath the thread? You would want to plan in any time for the clean up to occur. The other option you could do is wait on each thread to actually complete.
The reason for my comment asking if you could remove the detach call is because this method allows the main thread to exit and not care what was happening to any child threads. Instead, if you remove this call, and add:
$_->join() foreach threads->list();
to the end of your main block, this will require the main application to wait for each thread to actually complete.
If you leave the detach method in place, then you will need to sleep at the end of your code if you require your threads to perform any sort of clean-up. When you call detach on a thread, what you are telling Perl is that you don't care what the thread is doing when your main thread exits. If the main thread exits and there are threads that still running that have been detached, then the program will finish with no warnings. However, if you don't require any clean-up, and you still call detach, feel free to exit whenever you like.
Try out something like this....
#!/usr/bin/perl
use threads;
use threads::shared;
$|=1;
my ($global):shared;
my (#threads);
push(#threads, threads->new(\&mySub,1));
push(#threads, threads->new(\&mySub,2));
push(#threads, threads->new(\&mySub,3));
$i = 0;
foreach my $myThread(#threads)
{
my #ReturnData = $myTread->join ;
print "Thread $i returned: #ReturnData\n";
$i++;
}
sub mySub
{
my ($threadID) = #_;
for(0..1000)
{
$global++;
print "Thread ID: $threadID >> $_ >> GLB: $global\n";
sleep(1);
}
return( $id );
}
Is there any way to get alarm (or some other timeout mechanism) working in perl (>=5.012) threads?
Run alarm in your main thread, with a signal handler that signals your active threads.
use threads;
$t1 = threads->create( \&thread_that_might_hang );
$t2 = threads->create( \&thread_that_might_hang );
$SIG{ALRM} = sub {
if ($t1->is_running) { $t1->kill('ALRM'); }
if ($t2->is_running) { $t2->kill('ALRM'); }
};
alarm 60;
# $t1->join; $t2->join;
sleep 1 until $t1->is_joinable; $t1->join;
sleep 1 until $t2->is_joinable; $t2->join;
...
sub thread_that_might_hang {
$SIG{ALRM} = sub {
print threads->self->tid(), " got SIGALRM. Good bye.\n";
threads->exit(1);
};
... do something that might hang ...
}
If you need different alarms for each thread, look into a module that allows you to set multiple alarms like Alarm::Concurrent.
Edit: commentors point out threads::join interferes with SIGALRM, so you may need to test $thr->is_joinable rather than calling $thr->join
I have the following scenario:
sub_1 can run immediately
sub_2 can run immediately
sub_3 can run only after sub_1 finishes
sub_4 can run only after sub_1 finishes
sub_5 can run only after sub_2 finishes
sub_6 can run only after sub_2 finishes
sub_7 can run only after both sub_1 and sub_2 finish
sub_8 can run only after both sub_1 and sub_2 finish
I would like each sub to start run as soon as possible, than wait for all of them to finish.
I would really appreciate you help in creating a clean solution for this simple scenario -- I'm new to multi-threading.
I'm not sure if it makes a difference, but those subs are all in an object.
I'd suggest a "Boss/Worker" model, wherein one thread manages the subroutines to be executed in worker threads, who in turn report their status back to the boss upon completion.
In this model the boss is the only thread that needs to know how tasks are to be ordered. It might look something like this:
use threads;
use Thread::Queue;
use Thread::Pool;
our $done_queue = Thread::Queue->new;
our $work_pool = Thread::Pool->new;
sub sub_1 {
... do the work ...
$done_queue->enqueue('sub_1'); # tell the boss we're all done
}
sub sub_2 {
... do the work ...
$done_queue->enqueue('sub_2'); # tell boss we're done
}
...
# Main loop (boss thread)
$work_pool->enqueue(\&sub_1);
$work_pool->enqueue(\&sub_2);
while (my $sub_name = $done_queue->dequeue) {
# You, the boss thread, keep track of state and
# transitions however you like. You know what's
# just finished and what's finished in the past
...
}
Of course, abstraction can make that neater -- you could hide the Pool and the Queue behind a single object, one which didn't require sub_1() to know about the status queue at all:
$boss->enqueue( 'sub_1' => \&sub_1 ); # Will return 'sub_1' via await_completed()
$boss->enqueue( 'sub_2' => \&sub_2 ); # Will return 'sub_1'
while (my $sub_name = $boss->await_completed) {
...
}
Here's a possible solution using threads and thread sharing. Most of the code is just mocking up the test and emulating threads that have to do "work" before they finish. In the example the main thread spawns seven threads that each have a random amount of time that they have to do "work". The threads cannot begin working until the other threads they are dependent on (set in the dependencies array) have finished. You can change the thread dependencies and run the example a few times to illustrate that it works correctly.
Additionally you can have each thread terminate after it runs and have the main thread terminate after all of the subthreads have finished by checking the status hash.
use strict;
use warnings;
use threads;
use threads::shared;
my %status : shared;
my $dependencies = [
{3 => 1}, #three can only run after one has finished...
{4 => 1}, #four can only run after one has finished...
{5 => 2}, #five can only run after two has finished...
{6 => 1}, #etc...
{6 => 2},
{7 => 1},
{7 => 2}
];
main();
sub main{
foreach my $thread_number (1..7){
spawn_thread($thread_number);
}
while(1){
print "I am the main thread\n";
sleep(1);
}
}
sub spawn_thread{
my $thread_number = shift;
$status{$thread_number} = 'wait';
my $thr = threads->new(\&thread_routine, $thread_number);
}
sub thread_routine{
my $thread_number = shift;
my $working_time_left = int(rand(5)) + 1; #make a random time that this thread needs to "work"
while(1){
print "I am thread number $thread_number with status $status{$thread_number}\n";
{
lock(%status);
#see if this thread is active; if so, see if it finished running running
if ($status{$thread_number} eq 'active'){
if ($working_time_left <= 0){
$status{$thread_number} = 'ran';
}
}
else{
#see if we can activate
if ($status{$thread_number} eq 'wait'){
my $can_activate = 1;
foreach my $index (0..$#$dependencies){
if (exists $dependencies->[$index]->{$thread_number}){
if ($status{$dependencies->[$index]->{$thread_number}} ne 'ran'){
$can_activate = 0;
last;
}
}
}
if ($can_activate){
$status{$thread_number} = "active";
}
}
}
}
sleep(1);
if ($status{$thread_number} eq 'active'){ #do "work"
$working_time_left--;
}
}
}
Fork and create 2 processes:
In process 1:
sub_1; sub_3
In process 2:
sub_2; wait for sub_1 end; sub_4