Is there a way to read data from CGImage without internal caching?

Question

I am fighting with an internal caching (about 90 MB for 15 mp image ) in CGContextDrawImage/CGDataProviderCopyData functions.
Here is the stack-trace in profiler:

In all cases, IOSurface is created as a "cache", and isn't cleaned after @autoreleasepool is drained.
This leaves a very few chances for an app to survive.
Caching doesn't depend on image size: I tried to render 512x512, as well as 4500x512 and 4500x2500 (full-size) image chunks.

I use @autoreleasepool, CFGetRetainCount returns 1 for all CG-objects before cleaning them.

The code which manipulates the data:

+ (void)render11:(CIImage*)ciImage fromRect:(CGRect)roi toBitmap:(unsigned char*)bitmap {
    @autoreleasepool
    {
        int w = CGRectGetWidth(roi), h = CGRectGetHeight(roi);

        CIContext* ciContext = [CIContext contextWithOptions:nil];
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

        CGContextRef cgContext = CGBitmapContextCreate(bitmap, w, h,
                                                   8, w*4, colorSpace,
                                                   kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);


        CGImageRef cgImage = [ciContext createCGImage:ciImage
                                         fromRect:roi
                                           format:kCIFormatRGBA8
                                       colorSpace:colorSpace
                                         deferred:YES];


        CGContextDrawImage(cgContext, CGRectMake(0, 0, w, h), cgImage);

        assert( CFGetRetainCount(cgImage) == 1 );

        CGColorSpaceRelease(colorSpace);
        CGContextRelease(cgContext);
        CGImageRelease(cgImage);
    }
}

What I know about IOSurface: it's from the previously private framework IOSurface.
CIContext has a function render: ... toIOSurface:.
I've created my IOSurfaceRef and passed it to this function, and the internal implementation still creates its own surface, and doesn't clean it.

So, do you know (or assume):
1. Are there other ways to read CGImage's data buffer except CGContextDrawImage/CGDataProviderCopyData ?
2. Is there a way to disable caching at render?
3. Why does the caching happen?
4. Can I use some lower-level (while non-private) API to manually clean up system memory?

Any suggestions are welcome.

Answer 1

To answer your second question,

Is there a way to disable caching at render?

setting the environment variable CI_SURFACE_CACHE_CAPACITY to 0 will more-or-less disable the CIContext surface cache. Moreover, you can specify a custom (approximate) cache limit by setting that variable to a given value in bytes. For example, setting CI_SURFACE_CACHE_CAPACITY to 2147483648 specifies a 2 GiB surface cache limit.

Note it appears that all of a process's CIContext instances share a single surface cache. It does not appear to be possible to use separate caches per CIContext.

Answer 2

If you just need to manipulate CIImage data, may consider to use CIImageProcessorKernel to put data into CPU or GPU calculation without extracting them.

I notice that

[ciContext render:image toBitmap:bitmap rowBytes: w*4 bounds:image.extent format:kCIFormatRGBA8 colorSpace:colorSpace];

There is no such 90M cache. Maybe it's what you want.