AFNetworking - 6 operations at a time consume 80% of CPU - objective-c

Question

Can anybody answer to me, how come below code eats up whole core of a C2D 2.6GHz CPU?
It just downloads files in 10MB chunks, there can be like 600 of them, but the NSOperationQueue has a limit of 6 concurrent tasks.
How come the same app on Windows (written in C# eats only 2%, not 80%!), it is just a simple HTTP request!
for (DownloadFile *downloadFile in [download filesInTheDownload])
{
for (DownloadChunk *downloadChunk in [downloadFile chunksInTheFile])
{
NSString *downloadPath = [[NSString stringWithFormat:#"%#/%#", [download downloadFolder], [download escapedTitle]] stringByExpandingTildeInPath];
NSString *chunkPath = [downloadPath stringByAppendingFormat:#"/%#.%i", [downloadFile fileName], [downloadChunk chunkId]];
NSError *attributesError = nil;
NSDictionary *fileAttributes = [[NSFileManager defaultManager] attributesOfItemAtPath:chunkPath error:&attributesError];
NSNumber *fileSizeNumber = [fileAttributes objectForKey:NSFileSize];
uint64_t fileSize = [fileSizeNumber longLongValue];
NSLog(#"Chunk file size: %lli", fileSize);
uint64_t expectedSize = ([downloadChunk endingByte] - [downloadChunk startingByte]) + 1;
NSLog(#"Chunk expected size: %lli", expectedSize);
uint64_t newStartingByte = [downloadChunk startingByte] + fileSize;
if (fileSize == expectedSize)
{
NSLog(#"Chunk complete: %#.%i", [downloadFile fileName], [downloadChunk chunkId]);
}
else
{
NSURL *fileURL = [[NSURL alloc] initWithString:[downloadFile filePath]];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:fileURL];
NSLog(#"Normal range: %lli-%lli", [downloadChunk startingByte], [downloadChunk endingByte]);
NSString *range = [NSString stringWithFormat:#"bytes=%lli-%lli", newStartingByte, [downloadChunk endingByte]];
[request setValue:range forHTTPHeaderField:#"Range"];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.outputStream = [NSOutputStream outputStreamToFileAtPath:chunkPath append:YES];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
NSLog(#"%#", [NSString stringWithFormat:#"Chunk complete: %#.%i", [downloadFile fileName], [downloadChunk chunkId]]);
if (download.downloadedBytes == download.size)
[[NSNotificationCenter defaultCenter] postNotificationName:#"downloadFinished" object:download];
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"Error: %#", error);
}];
[operation setDownloadProgressBlock:^(NSInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {
download.downloadedBytes += bytesRead;
}];
[queue addOperation:operation];
}
}
}
Here are some time profiler screenshots, seems everything is because RunLoops if I read it correctly.

Answer 1

Seems like downgrading most of the properties to nonatomic seems to lower the usage from 80% to 20-30%. Now only downloadedBytes are atomic. What can I do also to make improvements?

Answer 2

In assuming the last progress block completes the download, you're sort of shoving a round peg down a square hole. I'm not sure if that's causing your high CPU usage or not, but I'd certainly fix that. AFNetworking has a function just for this, [AFHTTPRequestOperation setCompletionBlockWithSuccess:failure:], documented here. The correct block will be called based on the HTTP status code returned.
In other words, instead of this:
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.outputStream = [NSOutputStream outputStreamToFileAtPath:chunkPath append:YES];
[operation setDownloadProgressBlock:^(NSInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {
download.downloadedBytes += bytesRead;
if (download.downloadedBytes == download.size)
[[NSNotificationCenter defaultCenter] postNotificationName:#"downloadFinished" object:download];
}];
[queue addOperation:operation];
chunkId++;
Use this:
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.outputStream = [NSOutputStream outputStreamToFileAtPath:chunkPath append:YES];
[output setCompletionBlockWithSuccess: ^(AFHTTPRequestOperation *operation, id responseObject) {
[[NSNotificationCenter defaultCenter] postNotificationName:#"downloadFinished" object:download];
}
failure: ^(AFHTTPRequestOperation *operation, NSError *error) {
// you should probably do something here
}];
After that, I doubt you'd even need downloadedBytes. That removes your last atomic property, which should improve things.