4 ms·

Described algorithms ignore the ABA problem. The boost implementation avoids the ABA problem, but does not free up memory at all and stores pointers on 48 bits

by pebal 4y ago

Described algorithms ignore the ABA problem. The boost implementation avoids the ABA problem, but does not free up memory at all and stores pointers on 48 bits which is not enough on new architectures.

CyberDildonics 4y ago
Once again, you haven't mentioned at all how garbage collection changes anything, even though that was what you originally said and never backed up with anything. Described algorithms ignore the ABA problem. I literally wrote a method for doing that, an index with a version that can be checked to make sure nothing changed. Also if you're going to say that heavily tested implementations ignore the ABA problem you need to explain why you think that or why you think they won't work and again, why garbage collection changes anything. stores pointers on 48 bits which is not enough on new architectures. 48 bits is the size of the memory controller on modern CPUs and exceeding that would need over 281 terabytes of memory. Again, the original question is what lock free algorithm can be done with garbage collection that can't be done without it?
- pebal 4y ago
  You need to implement some form of GC when you want to free memory in a lock-free container. Read the thread and the employee's statements from Intel: https://community.intel.com/t5/Intel-oneAPI-Threading-Building/Concurrent-stack/td-p/900745 https://community.intel.com/t5/Intel-oneAPI-Threading-Buildi... New processors use 56 bits of virtual address. It doesn't matter if you have that much memory, because addresses are virtual. Also, newer versions of Android do not allow the use of unused address bits.
  - CyberDildonics 4y ago
    You need to implement some form of GC when you want to free memory in a lock-free container. This is again, your assertion, it isn't evidence or an explanation of any kind, you just keep saying the same thing. The link you have is people discussing a bunch of surrounding issues. Fundamentally, allocation of arbitrary memory just doesn't have to be ingrained in the lock free data structure. As soon as you can deal with 64 bits at a time, you can store pointers. There are lock free heap allocators and lock free block allocators that can be combined with whatever you are using to deal lock free with integers/pointers. Freeing memory is going to be a matter of ownership. If you pop a pointer, that thread should own it. Not only that, but a pointer combined with a reference count can always be used if necessary and again, 128 bit compare and swap has been around for 20 years.
    - pebal 4y ago
      In the link I pointed you have an example given and an explanation of why you need to have specific memory management for concurrent containers. It cannot be explained any clearer.
      - CyberDildonics 4y ago
        I've given you a lot of detailed explanations you haven't actually explained anything. I didn't see what you're talking about in the 15 year old message board discussion and I think if there was something specific and clear you would have copied and pasted it. I also think that if you had any understanding of what you're saying, you would have given an explanation yourself. So go ahead and actually put something here that you can back up. It is a common scenario where someone has no real evidence to link something adjacent and then tell someone to 'go find it in this link'.
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        pebal 4y ago
        From the document you linked: int lfstack_pop(_Atomic lfstack_t \*lfstack) { lfstack_t next; lfstack_t orig = atomic_load(lfstack); do { if (orig.head == NULL) // undefined behavior !!! { return -1; } next.head = orig.head->next; } while (!atomic_compare_exchange_weak(lfstack,&orig,next)); free(orig.head); return 0; } If the first thread is preempted before the if(...) is executed, and then the second thread executes the entire method, then you will use data after freeing when the first thread resumes. Consider why the boost container doesn't free memory.

by pebal 4y ago

CyberDildonics 4y ago

Once again, you haven't mentioned at all how garbage collection changes anything, even though that was what you originally said and never backed up with anything. Described algorithms ignore the ABA problem. I literally wrote a method for doing that, an index with a version that can be checked to make sure nothing changed. Also if you're going to say that heavily tested implementations ignore the ABA problem you need to explain why you think that or why you think they won't work and again, why garbage collection changes anything. stores pointers on 48 bits which is not enough on new architectures. 48 bits is the size of the memory controller on modern CPUs and exceeding that would need over 281 terabytes of memory. Again, the original question is what lock free algorithm can be done with garbage collection that can't be done without it?

pebal 4y ago
You need to implement some form of GC when you want to free memory in a lock-free container. Read the thread and the employee's statements from Intel: https://community.intel.com/t5/Intel-oneAPI-Threading-Building/Concurrent-stack/td-p/900745 https://community.intel.com/t5/Intel-oneAPI-Threading-Buildi... New processors use 56 bits of virtual address. It doesn't matter if you have that much memory, because addresses are virtual. Also, newer versions of Android do not allow the use of unused address bits.
- CyberDildonics 4y ago
  You need to implement some form of GC when you want to free memory in a lock-free container. This is again, your assertion, it isn't evidence or an explanation of any kind, you just keep saying the same thing. The link you have is people discussing a bunch of surrounding issues. Fundamentally, allocation of arbitrary memory just doesn't have to be ingrained in the lock free data structure. As soon as you can deal with 64 bits at a time, you can store pointers. There are lock free heap allocators and lock free block allocators that can be combined with whatever you are using to deal lock free with integers/pointers. Freeing memory is going to be a matter of ownership. If you pop a pointer, that thread should own it. Not only that, but a pointer combined with a reference count can always be used if necessary and again, 128 bit compare and swap has been around for 20 years.
  - pebal 4y ago
    In the link I pointed you have an example given and an explanation of why you need to have specific memory management for concurrent containers. It cannot be explained any clearer.
    - CyberDildonics 4y ago
      I've given you a lot of detailed explanations you haven't actually explained anything. I didn't see what you're talking about in the 15 year old message board discussion and I think if there was something specific and clear you would have copied and pasted it. I also think that if you had any understanding of what you're saying, you would have given an explanation yourself. So go ahead and actually put something here that you can back up. It is a common scenario where someone has no real evidence to link something adjacent and then tell someone to 'go find it in this link'.
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      - pebal 4y ago
        From the document you linked: int lfstack_pop(_Atomic lfstack_t \*lfstack) { lfstack_t next; lfstack_t orig = atomic_load(lfstack); do { if (orig.head == NULL) // undefined behavior !!! { return -1; } next.head = orig.head->next; } while (!atomic_compare_exchange_weak(lfstack,&orig,next)); free(orig.head); return 0; } If the first thread is preempted before the if(...) is executed, and then the second thread executes the entire method, then you will use data after freeing when the first thread resumes. Consider why the boost container doesn't free memory.