> On Aug 10, 2016, at 4:49 PM, Graham Cox <graham....@bigpond.com> wrote: > > >> On 9 Aug 2016, at 4:05 AM, Andrew Keller <and...@kellerfarm.com> wrote: >> >> In my app, I’m creating thumbnails of images. To do this in parallel, I’m >> using the global background dispatch queue: > > > Just to throw another consideration into the discusion, you don’t say what > the thumbnails are being used for. > > Typically you might want to display these to the user to browse a collection > of images. If that’s the case here, then it might be much more performant to > create them only as needed for display, rather than generate a huge batch of > them ahead of time. In other words, when an image thumbnail is required *for > display*, kick off a thread to fetch it (or display the one you have cached > already). This means that first time the user scrolls through the collection, > thumbnails will ‘pop in’ to view as each thread completes, but this effect is > usually easily tolerated. With scroll pre-fetching you might not even see it > at all. > > This way your threads are naturally limited by the number of images that are > shown at a time, or scrolled newly into view. > > —Graham
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