[boost] Re: Smart Pointer Timing Test Results

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2000-01-31 13:59:47

From: Miki Jovanovic <miki_at_[hidden]>
> "greg colvin" <gcolvi-_at_[hidden]> wrote: 
> > What is sad here is how easily Dave could hack up an allocator that is
> > way faster than the system defaults.  There is no reason, given what we
> > know about memory allocation, for the system allocator to be any slower
> > than what Dave did, but vendors keep on shipping crap.
> 
> The most important problem (for me) with using heap for storing ref
> counters, ptrs or anything is this: thread safety. To make it thread
> safe, everyone has to heavily protect their memory managers. Now, if
> you have lots of small units, smart pointers, lists, temporary objects
> all of them accessing heap, all of their memory allocations will have
> to be serialised, meaning only one thread at a time being allowed to
> allocate memory. This makes the memory manager a serious bottleneck in
> a real application.

You can write a thread-safe allocator with very low synchronization
overhead.  All that needs to be serialized is pushing and poppping
a pointer on a free list, for which there are tricks on most chips,
such as the cmpxchg instruction on Intel.

So far boost has followed the C/C++ library tradition of ignoring
threading issues.  I don't think we can get away with that for much
longer, but no one seems to have time and patience enough for the
assembly-level hacking it takes to do it right.

> So, while I am looking for a good/better smart pointer, I still use
> intrusive smart pointer in speed critical objects (also taking into
> account memory fragmentation from keeping the object and the reference
> counter separate).

The intrusive approach is almost sure to have the best performance,
but the results so far show the non-intrusive approaches to be close
enough for most purposes.

> Linked pointer seemed like a silver bullet, because it bypassed the
> memory manager, and still did not have big overheads, but now I see its
> drawbacks too. It is important to keep this conversation going just so
> as many facts and theories about the different implementations are
> discovered.

Agreed.

> Cheers,
> 
> Miki Jovanovic.
> 
> 
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