Hello everyone I think I have some memory leak in my Ktor ba

Question

Hello everyone I think I have some memory leak in my Ktor based application Tomcat is spawning many Threads and never releases them Is there any way to hook into moment when Ktor requests new Thread Some parts of application are working fine I can create hundreds requests and none Tomcat thread is created Or maybe this is bad idea and I should log all currently working threads Or do you suggest any other way of checking this issue

Rafał Kuźmiński · Answer

When I checked logs I can see that bouth OkHttp client and DefaultDispatcher threads aren t closed `org apache catalina loader WebappClassLoaderBase clearReferencesThreads The web application treespot dashboard 0 4 4 appears to have started a thread named DefaultDispatcher worker 10 but has failed to stop it This is very likely to create a memory leak Stack trace of thread ` `java base 11 0 9 1 jdk internal misc Unsafe park Native Method ` `java base 11 0 9 1 java util concurrent locks LockSupport parkNanos LockSupport java 357 ` `kotlinx coroutines scheduling CoroutineScheduler$Worker park CoroutineScheduler kt 783 ` `kotlinx coroutines scheduling CoroutineScheduler$Worker tryPark CoroutineScheduler kt 728 ` `kotlinx coroutines scheduling CoroutineScheduler$Worker runWorker CoroutineScheduler kt 711 ` `kotlinx coroutines scheduling CoroutineScheduler$Worker run CoroutineScheduler kt 665 ` `sorg apache catalina loader WebappClassLoaderBase clearReferencesThreads The web application treespot dashboard 0 4 4 appears to have started a thread named OkHttp Dispatcher but has failed to stop it This is very likely to create a memory leak Stack trace of thread ` `java base 11 0 9 1 jdk internal misc Unsafe park Native Method ` `java base 11 0 9 1 java util concurrent locks LockSupport parkNanos LockSupport java 234 ` `java base 11 0 9 1 java util concurrent SynchronousQueue$TransferStack awaitFulfill SynchronousQueue java 462 ` `java base 11 0 9 1 java util concurrent SynchronousQueue$TransferStack transfer SynchronousQueue java 361 ` `java base 11 0 9 1 java util concurrent SynchronousQueue poll SynchronousQueue java 937 ` `java base 11 0 9 1 java util concurrent ThreadPoolExecutor getTask ThreadPoolExecutor java 1053 ` `java base 11 0 9 1 java util concurrent ThreadPoolExecutor runWorker ThreadPoolExecutor java 1114 ` `java base 11 0 9 1 java util concurrent ThreadPoolExecutor$Worker run ThreadPoolExecutor java 628 ` `java base 11 0 9 1 java lang Thread run Thread java 834 `

tddmonkey · Answer

Why do you think you have a memory leak Purely based on what you have observed from your threads

Rafał Kuźmiński · Answer

Yes I have no other indicator but i am not that much experience in server development If I would refresh my web app like 1000 times Tomcat would stop responding because it wouldn t be able to create new Thread there is a limit for ~4000 threads

tddmonkey · Answer

What do you mean by refresh Do you mean hot reload your webapps

Rafał Kuźmiński · Answer

Ah no sorry I didn t describe it well enough This web application have some frontend that is connecting with backend through HTTP After i hit refresh on browser there a several requests to backend That s why after 1000 refreshes server would be full

Joost Klitsie · Answer

So which part of the application is spwaning the threads Server or Client

Joost Klitsie · Answer

if it is the client which I will assume as you mention OKHttp are you reusing the client or all the time creating a new one

Rafał Kuźmiński · Answer

< Joost Klitsie> I am not really sure That would be first thing that I have to check but I am not sure how I just checked Dependency container settings and HttpClient is created every time insted just once so maybe this is an issue

Joost Klitsie · Answer

I would say so

Rafał Kuźmiński · Answer

Yes it was it Thank you very much I had different HttpClient for OAuth and it was created every time for authorization Thanks wink

Rafał Kuźmiński · Answer

< Joost Klitsie> could you maybe tell me why this was happening Or point me to some docs that would explain why more than one HttpClient is a problem

Joost Klitsie · Answer

I am unsure as to when httpclients release their resources so spawning multiple will cost you in terms of initialization time and also you will probably have to do the clean up yourself Internally a client has a connection pool and will reuse this across different requests making it more efficient firing multiple calls to the same host So I would say the optimized way of reusing connections and of course the threads a client will spawn are a good reason to reuse a client throughout your application

Rafał Kuźmiński · Answer

I see I just found official docs about it <https ktor io docs client html> and you are right releasing resources should be done explict in code