[phobos] Fwd: Re: Ruling out arbitrary cost copy construction?

[Max Samukha]

On Wed, Nov 3, 2010 at 6:28 PM, Michel Fortin <michel.fortin at michelf.com>wrote:

> Le 2010-11-03 à 11:00, Max Samukha a écrit :
>
> > These notes on copy-constructible Qt types may be useful for the
> > discussion:
> >
> > 1. 1/5 (approx. 100 classes) of all classes in core, gui, network,
> webkit,
> > svg and opengl packages define public copy-constructors.
> >
> > A half of those are in reference-counted COW types (approx. 50 classes).
> The
> > remaining 50 classes are reference-counted types without COW, types with
> > allocating copy-constructors and types with trivial non-allocating
> > constructors.
> >
> > Most of the types with allocating copy-constructors I would probably
> > implemented as classes in D. Polymorphic types like QListWidgetItem that
> > provide the copy-constructor only for clone() reimplementation should
> > definitely be classes in D.
>
> This prompted me at looking into how reference counting works in Cocoa.
> Note that all Cocoa objects use reference semantics and are reference
> counted.
>
> The reference counter is not stored as part of the object. Instead,
> reference counters are scattered among 8 global hash tables based on bits
> 8,9,10 of the pointer value. Each table has its own spinlock which is used
> for synchronization. On embeded system (iOS presumably), there is only one
> hash table instead of 8. I guess having many tables is worthless if you have
> only one core.
>
> I haven't done any benchmarking, but it seems the assumption is that each
> thread will use a different memory region that will hopefully fall in a
> different hash table and thus under a different spinlock and cacheline,
> which should reduce contention.
>
> Relevant source code (under the Apple Public Source License):
>
> See function __CFDoExternRefOperation in
> <http://www.opensource.apple.com/source/CF/CF-550/CFRuntime.c>
>
> Spinlock implementation it uses:
> <
> http://google.com/codesearch/p?hl=fr#pFm0LxzAWvs/darwinsource/tarballs/apsl/Libc-391.tar.gz%7Cz8mNFiEo9vA/Libc-391/i386/sys/OSAtomic.s&q=OSSpinLock%20package:darwin
> >
>
>
An interesting approach. I wonder how well it performs compared to
traditional reference counters.


>
> --
> Michel Fortin
> michel.fortin at michelf.com
> http://michelf.com/
>
>
>
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