Fast NSData unarchiving - ios

Question

I create NSData from a class instance like this:
NSData* data = [NSKeyedArchiver archivedDataWithRootObject:db];
[data writeToFile:#"/Users/.../db.data" atomically:true];
Then I need to load this instance:
NSURL* dbUrl = [[NSBundle mainBundle]URLForResource:#"db" withExtension:#"data"];
NSData* dbData = [NSData dataWithContentsOfURL:dbUrl];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
self.db = [NSKeyedUnarchiver unarchiveObjectWithData:dbData];
});
This loading lasts cca. 6 seconds for ~70MB file size. It is very slow! I tried to separate db instance into 2 different parts (~35MB) and load them like this:
self.db = [[Database alloc]init];
NSURL* dbUrl1 = [[NSBundle mainBundle]URLForResource:#"db1" withExtension:#"data"];
NSData* dbData1 = [NSData dataWithContentsOfURL:dbUrl1];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
self.db.nodes = [NSKeyedUnarchiver unarchiveObjectWithData:dbData1];
});
NSURL* dbUrl2 = [[NSBundle mainBundle]URLForResource:#"db2" withExtension:#"data"];
NSData* dbData2 = [NSData dataWithContentsOfURL:dbUrl2];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
self.db.trips = [NSKeyedUnarchiver unarchiveObjectWithData:dbData2];
});
With this way loading lasts cca. 3 seconds (2x faster). Do you know another ways to load it faster?
EDIT 1:
I'm not sure that it can help, but archived object has two NSMutableArray instances. The first one contains a lot of Node objects:
#interface DatabaseNode : NSObject <NSCoding>
#property (assign, nonatomic) NSInteger index;
#property (strong, nonatomic) NSString* name;
#property (assign, nonatomic) double lat;
#property (assign, nonatomic) double lon;
#end
And the second one contains a lot of Trip objects:
#interface DatabaseTrip : NSObject <NSCoding>
#property (assign, nonatomic) NSInteger index;
#property (strong, nonatomic) NSString* name;
#property (assign, nonatomic) NSInteger service;
#property (strong, nonatomic) NSMutableArray* departureTimes; //Contains NSDate
#end

Answer 1

Given that you need all the data in memory from the beginning of your app's session, you're going to have to go with a solution other than NSKeyedArchiver. NSKA is designed to archive complex object graphs and, thus, is optimized to solve a different problem.
In your case, I'd suggest a build phase that writes the static data to a file format that can be mapped into memory without parsing. Then, at runtime, map the file and have a pass that turns it into a minimal encapsulation in Foundation types.
For Strings, a string table like solution will work well enough. Dates can be a little bit trickier, maybe. I'd test to see if the time interval based creation methods are fast. If they are, then you can just store your dates as arrays (not NSArrays, but flat C arrays of NSTimeIntervals) in the mapped file and do a fixup pass.
Of course, you may also skip the fixup of some subset of data based on the access patterns in your app. If your users don't really look at all the dates at once, then there is no need to fix 'em up on load, but they can be created on demand.