CODEWITHSUNDEEP
c++multithreadingasynchronous
user10146018
user10146018

Reputation:

How to execute code asynchronously without creating new threads

I am using Qt SQL which is blocking API so I have to execute SQL code in Separate thread (QtConcurrent::run) and return (Q)future.

something like this:-

QFuture<QString> future = QtConcurrent::run( []() { /* some SQL code */ } );

auto  watcher = new QFutureWatcher<QString>();

watcher.setFuture(future);

connect(watcher,&QFutureWatcher<QString>::finished,
               [future](){ /* code to execute after future is finished */ });

But I learned that Threading is costly. every context switch is expensive. So it looks like CPU wastage to create new Thread just for waiting for result from MySQL server. My application is going to run on single core Virtual Machine on Google Cloud anyways . it there any way I can execute Qt SQL code asynchronusly without possibly creating new thread ?

I was also wondering how other APIs like Qt Networking implement asynchronus API without create new thread ? or i am wrong and they do create new thread under the hood ?

Upvotes: 3

Views: 384

Answers (2)

userbb
userbb

Reputation: 1874

By using QtConcurrent::run you already solved one problem - cost of creating thread because it use a thread pool.

When comes to context switches, first you could try to measure them with perf stat. And depends on situation, optimize it. If its just simple queries then probably vast majority of context switches comes from the system, not your app.

Doing something async means that you can execute task and move forward with your current code without waiting for results. But usually such task i.e sql query will spawn thread/process or will make request to OS.

Qt Networking make i.e read request and OS signals (epoll) when data will arrive. But in case of single core OS will interrupt your thread anyway.

If you have many many small queries you could try optimize them to make less queries, do caching.

Upvotes: 2

stark
stark

Reputation: 13189

Many threaded applications run on a single core. Flushing cache to run on a separate core is also expensive. Use the right tool for the job. There's nothing wrong with threads.

That said, if you really want to run on a single thread use a workqueue to keep track of async task progress. The libevent library does this for you, but there are others. You just run a polling loop adding work onto the queue and executing callbacks when a task needs attention or completes.

Upvotes: 2

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