QtConcurrent::map uses 100% of CPU

gronerth

Hello!!

I was implementing a project where i have to do snmp queries periodically (it's up to the interval defined). Because, it could take too much time to do all the queries, i am using QtConcurrent::map to do the queries in another thread but i can see that the cpu reaches 100% while the program it's doing the queries....is it normal that the cpu reaches 100% when you use threads? how can you avoid that?

Thanks in advance..

giesbert

If threads are doing nothing they should require 0% cpu.
But the point is: what is doing nothing?

If you create your own threads, yo normally wait on wait conditions. This resumes the thread and lets it sleep till the condition is meat.
Regarding QtConcurrent, it means it depends what your function is doing.
Please show us some code, then we can perhaps answer your question.

ZapB

As Gerolf said it depends upon what you are asking to be done in the worker threads. Post some code...

gronerth

Well, i have the function "executeAllActiveTasks" that takes a string as an argument and calls a function "requestandsaveData", this function does a snmp-get to an specific device and waits until the data is available and then saves the data in a file. The following is the code for "executeAllActiveTasks"

@void executeAllActiveTasks(QString tname)
{
// double val;
QDir path( "./databases/" );

task *t = tasks.value(tname);
RCDevice *d = devices.value(t->device_id());

       if(t!=NULL || t->status()!=STARTED || d!=NULL)
       {
           Metrics.value(t->metricType())->requestAndSaveData(d,path.absolutePath()+ QString("/") +t->taskName()+QString(".rrd"),t->metricType(),t->metricParams(),0);
       }

}@

I call that function with the following code, where "taskActivesByThread" it is a QStringList...

@watcher.setFuture(QtConcurrent::map(tasksActivesByThread,executeAllActiveTasks));
@

giesbert

how long running is his task?

gronerth

It is up to the size of the "taskActivesByThread" and the time to get the response of the devices, it could be one minute or even more. The application works fine, but I just want to know if any other alternative methods would produce 100% of CPU while the polling is active...

baysmith

Possibly something is in an active wait state, meaning it is constantly querying to see if the results are ready.

gronerth

Well, yesss..they are waiting for the results to be ready, but just because a thread is in an active state will produce a huge increment in the cpu?

baysmith

If the thread is in an active wait state (for example waiting for a spin lock), yes. If the thread is in an inactive wait state (for example waiting for a conditional variable), no.

What does snmp-get do and is it thread safe?

gronerth

It is thread safe...the snmp get is really a "QProcess" calling the libraries of the net-snmp, so i need the wait until the "QProcess" finish to captutre the data...

baysmith

How is it waiting for the QProcess to finish and how is it capturing the data?

gronerth

It's something like this...

@
QProcess snmpGet;
snmpGet.start(program,params);

if (snmpGet.waitForStarted())
{
if (!snmpGet.waitForFinished())
{
return INVALID_VALUE;
}
QString output(snmpGet.readAll());
//Then i have to parsing the data to obtain the data...

}else{

return INVALID_VALUE
}

@

baysmith

Nothing seems wrong with what you've described so far. If you interrupt it in a debugger when the CPU is at 100%, where does it break?

ZapB

Profile it and see where the profiler says the cpu is spending all of its time.