Hello everyone,
We are in the process of migrating a high-performance storage KEXT to DriverKit. During our initial validation phase, we noticed a performance gap between the DEXT and the KEXT, which prompted us to try and optimize our I/O handling process.
Background and Motivation:
Our test hardware is a RAID 0 array of two HDDs. According to AJA System Test, our legacy KEXT achieves a write speed of about 645 MB/s on this hardware, whereas the new DEXT reaches about 565 MB/s. We suspect the primary reason for this performance gap might be that the DEXT, by default, uses a serial work-loop to submit I/O commands, which fails to fully leverage the parallelism of the hardware array.
Therefore, to eliminate this bottleneck and improve performance, we configured a dedicated parallel dispatch queue (MyParallelIOQueue) for the UserProcessParallelTask method.
However, during our implementation attempt, we encountered a critical issue that caused a system-wide crash.
The Operation Causing the Panic:
We configured MyParallelIOQueue using the following combination of methods:
In the .iig file: We appended the QUEUENAME(MyParallelIOQueue) macro after the override keyword of the UserProcessParallelTask method declaration.
In the .cpp file: We manually created a queue with the same name by calling the IODispatchQueue::Create() function within our UserInitializeController method.
The Result:
This results in a macOS kernel panic during the DEXT loading process, forcing the user to perform a hard reboot.
After the reboot, checking with the systemextensionsctl list command reveals the DEXT's status as [activated waiting for user], which indicates that it encountered an unrecoverable, fatal error during its initialization.
Key Code Snippets to Reproduce the Panic:
In .iig file - this was our exact implementation:
class DRV_MAIN_CLASS_NAME: public IOUserSCSIParallelInterfaceController
{
public:
virtual kern_return_t UserProcessParallelTask(...) override
QUEUENAME(MyParallelIOQueue);
};
In .h file:
struct DRV_MAIN_CLASS_NAME_IVars {
// ...
IODispatchQueue* MyParallelIOQueue;
};
In UserInitializeController implementation:
kern_return_t
IMPL(DRV_MAIN_CLASS_NAME, UserInitializeController)
{
// ...
// We also included code to manually create the queue.
kern_return_t ret = IODispatchQueue::Create("MyParallelIOQueue",
kIODispatchQueueReentrant,
0,
&ivars->MyParallelIOQueue);
if (ret != kIOReturnSuccess) {
// ... error handling ...
}
// ...
return kIOReturnSuccess;
}
Our Question:
What is the officially recommended and most stable method for configuring UserProcessParallelTask_Impl() to use a parallel I/O queue?
Clarifying this is crucial for all developers pursuing high-performance storage solutions with DriverKit. Any explanation or guidance would be greatly appreciated.
Best Regards,
Charles
