Why would using Swift DispatchGroup load CPU to 1000%?

Question

Joanna Carter OP

Created Oct ’20

Replies 1

Boosts 0

Participants 2

I have some code that calls an external library asynchronously to read EXIF keywords from image files :

Code Block   func readKeywords()
  {
    keywordsForSelectedItems.removeAll()
    
    let group: DispatchGroup = .init()
    
    let queue: DispatchQueue = .init(label: "com.keywords")
    
    collectionView.selectionIndexPaths.forEach
    {
      indexPath in
      
      let item = self.collectionView.item(at: indexPath) as! CollectionViewItem
      
      let url = item.url!
      
      group.enter()
      
      queue.async(group: group)
      {
        do
        {
          try ExifReader.keywords(for: url)
          {
            keywords in
            
            if let keywords = keywords,
               keywords.count > 0
            {
              self.keywordsForSelectedItems.append(keywords)
            }
            
            group.leave()
          }
        }
        catch
        {
          print(error.localizedDescription)
        }
      }
    }
    group.notify(queue: .main)
    {
      let firstValue: [String]? = self.keywordsForSelectedItems.first
      {
        keywords in
        
        keywords.count > 0
      }
        
      if firstValue != nil
      {
        let keywords = self.keywordsForSelectedItems.reduce(Set(firstValue!))
        {
          result, item in
          
          result.union(item)
        }
        
        self.tokenView.objectValue = keywords.sorted()
      }
    }
  }

Before I added this code, everything worked fine and images were displayed perfectly in a CollectionView.

But, since adding this code, as soon as this code has been called, the CPU fan starts to run, anywhere up to 1000% at times and it can block other asynchronous processes that normally load images in the background.

Anyone got any ideas?

Boost

Answer 1

OOPer OP

Oct ’20

I cannot say anything sure as you are not showing enough code to reproduce the issue.

But at least you need to fix one thing with this line:

Code Block               self.keywordsForSelectedItems.append(keywords)

I guess keywordsForSelectedItems is a Swift Array, which is not thread-safe.

You should not access a Swift Array from multiple threads.

You may need to prepare a serial queue for the Array, and access it only within the queue.

0