Re: C++ Memory Management Innovation: GC Allocator

"Chris Thomasson" <>
Thu, 24 Apr 2008 01:43:35 CST
"xushiwei" <> wrote in message

The difference between StreamFlow and ScopeAlloc (or AutoFreeAlloc)

1. StreamFlow uses thread local storage, while ScopeAlloc doesn't.

There are benefits for using TLS. Think of a simple contrived scenario like:

void function() {
 // I need to allocate from m_alloc...
 // How can I do this without adding any parameters?

void thread() {
 GenericAllocator m_alloc;

AFAICT, your allocator can use TLS as-is... Basically, something like:

void function() {
 GenericAllocator* const pm_alloc = pthread_getspecific(...);
 // Now I can allocate from m_alloc! :^D

void thread() {
 GenericAllocator m_alloc;
 pthread_setspecific(..., &m_alloc);

Don't you think that your design could "possibly" benefit from using TLS?
IMVHO, it would increase its flexibility...

2. StreamFlow provides global allocation procedures, while ScopeAlloc
uses non-static allocator instances to allocate memory.

StreamFlow directs global allocation function's directly to the
calling threads heap... Something like:

void* malloc(size_t sz) {
 PerThreadHeap* const _thisheap = pthread_getspecific(...);
 return _thisheap->allocate(sz);

void free(void* ptr) {
 PerThreadHeap* const _thisheap = pthread_getspecific(...);

Using global allocation functions does not limit performance in
any way, shape or form.

3. ScopeAlloc forbid you to deallocate memory by yourself.

IMVHO, many low-level programs "need" the flexibility to be able
to control exactly when a piece of memory should be returned to
the system.

GC Allocator doesn't forbid you to create in thread A and free in
thread B. But it is not recommended. However, GC Allocator forbid you
to deallocate memory manually. So, if you implement algorithms that
are related to t (that is, how many time of the algorithm spent is
unsure.), GC Allocator doesn't fit you directly.

What does the algorihtm look like for remote deallocations? Here is
how I do it in my vZOOM allocator:
- create a thread-local instance of a user-defined single-threaded
allocator in every thread (e.g., ms heap w/ HEAP_NO_SERIALIZE).

- allocation requests are forwarded to this thread-local user allocator

- if free request goes from thread that allocated block (e.g., the origin
thread), then free request is forwarded to this thread-local user allocator.

- if free request goes from another thread, then you accumulate this block
in per-thread stack-based freelist "belonging to the origin thread", using
single atomic CAS.

- blocks from this freelist is actually reused/freed in batches using
single atomic SWAP when thread allocates/deallocates another block. For
instance, a thread that fails to allocate from its thread-local heap will do
a SWAP on the freelist and try and fulfill the allocation request from

My algorithm works with basically any single-threaded allocator, AFAICT,
it would even work with your existing code. You would not need to change
anything. In fact, and end-user could plug your code into this algorithm,
and it would just work.

Any thoughts?


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