Oleg Andreev
Month
“Tracing and reference counting are uniformly viewed as being fundamentally different approaches to garbage collection that possess very distinct performance properties. We have implemented high-performance collectors of both types, and in the process observed that the more we optimized them, the more similarly they behaved — that they seem to share some deep structure.” David F. Bacon at al.
Great article explaining multithreading issues on multiprocessor systems.
Reference-counting is traditionally considered unsuitable for multi-processor systems. According to conventional wisdom, the update of reference slots and reference-counts requires atomic or synchronized operations. In this work we demonstrate this is not the case by presenting a novel reference-counting algorithm suitable for a multi-processor system that does not require any syn- chronized operation in its write barrier (not even a compare-and-swap type of synchronization). A second novelty of this algorithm is that it allows eliminating a large fraction of the reference- count updates, thus, drastically reducing the reference-counting traditional overhead. This paper includes a full proof of the algorithm showing that it is safe (does not reclaim live objects) and live (eventually reclaims all unreachable objects).
1. Some GCs do pointer recognition in the arbitrary data array (e.g. Boehm-Demers-Weiser GC); this is not necessary if GC should track objects of the known structure (e.g. Steve Dekorte’s GC)
2. If we track object references only, there’s no need to fight fragmentation of the GC-managed heap: all entries are of the same size.
3. After coding Obj-C for a while, I’ve noticed that the only issue which should be resolved by some kind of garbage collector is circular references. Retained properties and autorelease pools already help to avoid manual retain/release calls. That is: allocation should be always succeeded by autorelease and all the properties should be nullified on deallocation (this could be done automatically).
I wonder if it is possible to use a simple reference-counting mechanism with a simple referential cycles resolution, thinking that way we can imagine a very simple and efficient garbage collector.
References:
1. Minimizing Reference Count Updating with Deferred and
Anchored Pointers for Functional Data Structures by Henry Baker
2. Concurrent Cycle Collection in Reference Counted Systems by David F. Bacon and V.T. Rajan
1. When using sandbox user account, do not sign in using Preferences, just sign out and sign in when your app asks so.
2. Transaction receipt should be Base64 encoded before sending to Apple verification service. This is not mentioned in the docs.
3. Subscriptions should not be restored using restore API. Do not even try: Apple suggests sending relevant data from your application server.
4. Title/description you set in iTunes connect do not really matter if you supply products from the server. Only product id and the price tier matter. Everything else you supply by yourself.
5. Do not forget to sort the products elsewhere since it is impossible to sort them on iTunes Connect site.
6. Transaction receipt should be verified at apple server for two reasons: 1) secure validation; 2) this is the only way to get transaction details