What’s the CPU utilization? Are you actually getting full use of them, or are your threads blocked waiting for something?
On Apr 24, 2014, at 11:14 PM, Jens Alfke <j...@mooseyard.com> wrote: > 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 > _______________________________________________ > > Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) > > Please do not post admin requests or moderator comments to the list. > Contact the moderators at cocoa-dev-admins(at)lists.apple.com > > Help/Unsubscribe/Update your Subscription: > https://lists.apple.com/mailman/options/cocoa-dev/dave.fernandes%40utoronto.ca > > This email sent to dave.fernan...@utoronto.ca _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
