Xcode 14 beta: Thread running at QOS_CLASS_USER_INTERACTIVE waiting on a lower QoS thread running at QOS_CLASS_DEFAULT. Investigate ways to avoid priority inversions

[groue]

Thread running at QOS_CLASS_USER_INTERACTIVE waiting on a lower QoS thread running at QOS_CLASS_DEFAULT. Investigate ways to avoid priority inversions

This message can appear as a runtime warning (purple ⚠️ sign), in Xcode 14 beta, inside the internal GRDB.Pool.get() method:

final class Pool<T> {
    func get() throws -> (element: T, release: () -> Void) {
        try barrierQueue.sync {
            itemsSemaphore.wait() // <- ⚠️ Thread running at QOS_CLASS_USER_INTERACTIVE…

It happens when the application performs a synchronous database read, from the main thread, with DatabasePool.

---

To remove this warning, instruct the database to run with higher priority:

var configuration = Configuration()
// As we plan to access the database from the main thread, make
// sure it runs with an appropriate quality of service:
configuration.qos = .userInitiated
let dbPool = try DatabasePool(path: ..., configuration: configuration)

---

This warning is a new tool in Xcode 14 beta, the Thread Performance Checker. You enable it in Xcode > Edit Scheme… > Run > Diagnostics > Runtime API checking > check "Thread Performance Checker".

Thank you Apple for warning us that we misuse our threads :-)

---

We'll probably raise the default quality of service of GRDB databases in a future release. If we do not, many Xcode 14 users will start to see this runtime warning, and I don't want to ship a library that warns on first use:

• I do not want to deal with the support requests this would create.
• SQLite is very fast, simple apps ought to remain simple, and nobody should be shamed for accessing the db from the main thread.

Please feel free to comment below, if you think we can be more subtle than raising all databases to .userInitiated 😅

[groue]

Fixed by #1235.

[groue]

The fix has shipped in v5.25.0

[peterhoucll]

Hi, I upgraded GRDB to 6.0.0 and the problem is not solved in Xcode 14.

[groue]

Hello @peterhoucll. Please provide a reproducing sample code, if you have time. This would greatly help.

[peterhoucll]

Hi, after checking, the warning will appear only in the simulator, thanks.

[groue]

All right @peterhoucll. This remains an interesting warning, even if it only happens in the simulator. Again, only if you have the time, and you can see that the warning happens from within GRDB-related code, please share.

[yeahphil]

Hello @groue, I'm still able to repro this in the simulator AND on devices using GRDB 6.0.0. Here's a brief description of where I'm seeing the warning. I haven't tried to put together a sample project that reproduces the issue, but let me know if that would be helpful and I'd be happy to do so.

We see this occur when using DatabasePool and calling read(). The warning comes from Pool.get(), at this DispatchSemaphore.wait() call:

    func get() throws -> (element: T, release: () -> Void) {
        try barrierQueue.sync {
            itemsSemaphore.wait() // <--- "Thread Performance Checker: Thread running at QOS_CLASS_USER_INTERACTIVE waiting on a lower QoS thread running at QOS_CLASS_DEFAULT."
            itemsGroup.enter()
            // ...method continues

That's happening for a tiny minority of reads, and I'm sure there other conditions necessary for it to warn. Again if it would be helpful, I'd be happy to investigate further.