didDropSampleBuffer is called very often in iOS - ios

Question

I capture the video and do some analyses on it in captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer delegate. but after a short time this method is not called. then captureOutput:(AVCaptureOutput *)output didDropSampleBuffer delegate is called.
When I don't do anything in didOutputSampleBuffer, everything is okay.
I run a tensor flow model in this delegate. And this causes the problem.
Problem:
The problem is that when didDropSampleBuffer is called, didOutputSampleBuffer will not called again.
My solution: My solution was stoping and starting avCaptureSession. but that caused extra memory usage! Which finally caused my app to crash.
- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
// ****** do heavy work in this delegate *********
graph = [TensorflowGraph new];
predictions = [graph runModelOnPixelBuffer:pixelBuffer orientation: UIDeviceOrientationPortrait CardRect: _vwRect];
}
- (void)captureOutput:(AVCaptureOutput *)output didDropSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
CFTypeRef droppedFrameReason = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_DroppedFrameReason, NULL);
NSLog(#"dropped frame, reason: %#", droppedFrameReason);
}
----> dropped frame, reason: OutOfBuffers
According to [https://developer.apple.com/library/archive/technotes/tn2445/_index.html]:
This condition is typically caused by the client holding onto buffers
for too long, and can be alleviated by returning buffers to the
provider.
How can I return buffer to the provider?
Edited
After 11 times that CGImageRef cgImage = [context createCGImage:resized fromRect:resized.extent]; line is executed, The didDropSampleBuffer is called. commenting CFRelease(pixelBuffer) has no difference in result. Does it means that pixelBuffer is not released?
CFRetain(pixelBuffer);
CVPixelBufferLockBaseAddress(pixelBuffer, kCVPixelBufferLock_ReadOnly);
CIImage* ciImage = [[CIImage alloc] initWithCVPixelBuffer:pixelBuffer];
ciImage = [ciImage imageByCroppingToRect:cropRect];
CGAffineTransform transform = CGAffineTransformIdentity;
CGFloat angle = 0.0;
transform = CGAffineTransformRotate(transform, angle);
CIImage* resized = [ciImage imageByApplyingTransform:transform];
CIContext *context = [CIContext contextWithOptions:nil];
CGImageRef cgImage = [context createCGImage:resized fromRect:resized.extent]; // *********************************
UIImage* _res = [[UIImage alloc] initWithCGImage:cgImage];
CFRelease(pixelBuffer);
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