GCD killed my performance
GCD killed my performance
- Subject: GCD killed my performance
- From: Jens Alfke <email@hidden>
- Date: Thu, 24 Apr 2014 20:14:35 -0700
I’m writing an Objective-C API around a database library, and trying to add some optimizations. There’s a lot of room for parallelizing, since tasks like indexing involve a combination of I/O-bound and CPU-bound operations. As a first step, I made my API thread-safe by creating a dispatch queue and wrapping the C database calls in dispatch_sync blocks. Then I did some reorganization of the code so different parts run on different queues, allowing I/O and computation to run in parallel.
On my MacBook Pro this gave me a nice speedup of 50% or more.
But when I tested the code on my iPhone 5 today, I found performance had dropped by about a third. Profiling shows that most of the time is being spent in thread/queue management or Objective-C refcount bookkeeping. It looks as though adding GCD introduced a lot of CPU overhead, and the two cores on my iPhone aren’t enough to make up for that, while the eight cores in my MacBook Pro make it worthwhile.
I tried backing out all the restructuring of my code, so there’s no actual parallelism going on, just the dispatch_sync calls. Predictably, performance is even worse; slightly more than half as fast as without them.
So, I’m pretty disappointed. I know that dispatch queues aren’t free, but I wasn’t expecting them to be this expensive! I’m not doing anything silly like wrapping dispatch_sync around trivial calls. The APIs I’m using it on do things like reading and writing values from the persistent store. I was expecting the cost of thread-safety to be lost in the noise compared to that.
Any suggestions on what to try next?
—Jens
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