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POlock free CAS confusion
 

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    <p>Hey guys,<br> I am reading these so called non-blocking techniques, but i have few doubts :</p> <p>1) lock-free operation are performed using atomic operation, now what are these atomic operation ? i mean at certain level they also need to have a lock right ? so Does this lock-free approach provides us locking at finer granularity only ? <br> 2) they say non-blocking list, Now what a non-blocking list should be : if more than one threads comes at the same insertion, only one is going to get succeed, another one will do some other work right ?, but if other thread has no choice except inserting a node then how come it is non-blocking ? won't it will get blocked until previous one is done ?? <br> 3) In java, how do they atomic operation ? doesn't they do something like <code> synchronized boolean ..... </code> so how it is lock-free since they are acquiring lock i.e. synchronized section ? 4) CAS is usually used to implement atomic operation. So doesn't cas allow only one operation to happen on the same object , and stops ( <strong>block</strong> ) others those want to operate on the same object ? Sorry for so many doubts... please clarify...</p> <p><strong>EDIT</strong> what happens when i have a structure to update ? does it also supported by hardware ? No right, So doesn't language would be acquiring locks at some level to make these structure operation atomic ?<br> About <strong>JAVA</strong> : there are AtomicReference and AtomicReferenceFieldUpdater class which provides updation to a object ( structure or any kind of object) , so can we compare them in terms of performance i mean speed ? Both in tern uses Unsafe class which uses Native class . </p>
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    1. CONon blocking can be a lot faster (magnitudes) in the right situation. I have a skiplist that I made lock free. This is because 99,999 times out of 100,000, no two threads ever try to add to the list at the same time. This is even when running on an SMT system with many threads. The overhead of using a mutex or futex was a waste. I just detect a list change during an attempt to insert or delete....and try again. In our app, this is MUCH faster. There is blocking 1 time per million redos, when the thread changing the list was pre-empted in the change code.
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