Question

i’m new ios and i don’t know performance of NSThread an multithread on IOS.
I’ve a function

-(void) writeBlock:(NSString *)srcPath toPath:(NSString *)destPath fromIndex:(int)fIndex toIndex:(int)tIndex
{
    //create file handle for srcPath and destPath
    for(int i=fIndex;i<=tIndex;i++){
       //read from source filehandle
       //write to dest filehandle
    }
    //close filehandle.
}

If i create a single thread for above function,the time for perform this function is 1 second.
But when i create:

for(int i=0;i<20;i++)
{
    [NSThread detachNewThreadSelector: @selector(writeBlock:) toTarget:self withObject:myObject];
    //for each thread, tIndex-fIndex is the same with tIndex-fIndex of single above thread
}

The time for perform 20 threads is 13-16seconds.
I think the time will be 1-5 seconds.Multi thread is slow?
Can anyone help me?Thanks so much

Answer 1

Single-Core smartphone processors can only handle so much. Creating 20 threads on the iPhone will considerably slow performance down – creating threads is expensive.
Another option to creating a low-level NSThread is using an NSOperation or NSOperationQueue – see also this this thread on StackOverflow.

Moreover, writing/reading to/from memory can only be done by one thread at a time.