How did You get so smart ?
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amin...@gmail.com kirjutas Teisipäev, 24. detsember 2019 kl 23:04:40 UTC+2:
Hello,
Lock Versus Lock-Free..
The class of problems that can be solved by lock-free approaches is limited.
Furthermore, lock-free approaches can require restructuring a problem.
As soon as multiple shared data-structures are modified simultaneously,the only practical approach is to use a lock.
All lock-free dynamic-size data-structures using such CAA (Compare-and-assign) require some form of garbage collector to lazily delete storage when it is no longer referenced. In languages with garbage collection, this capability comes for free (at the
cost of garbage collection). For languages without garbage collection, the code is complex and error prone in comparison with locks, requiring epoch-based reclamation, read-copy-update (RCU), or hazard pointers.
While better performance is claimed for lock-free data-structures, there is no long-term evidence to support this claim. Many high-performance locking situations, e.g., operating system kernels and databases, continue to use locking in various forms,
even though there are a broad class of lock-free data-structure readily available.
While lock-free data-structures cannot have deadlock, there is seldom deadlock using locks for the simple class of problems solvable using lock-free approaches. For example, protecting basic data-structure operations with locks is usually very
straightforward. Normally deadlock occurs when accessing multiple resources simultaneously, which is not a class of problems
dealt with by lock-free approaches. Furthermore, disciplined lock usage, such as ranking locks to avoid deadlock, works well in practice and
is not onerous for the programmer.Finally, some static analysis tools are helpful for detecting deadlock scenarios.
Lock-free approaches have thread-kill tolerance, meaning no thread owns a lock, so any thread can terminate at an arbitrary point without leaving a lock in the closed state. However, within an application, thread kill is an unusual operation and thread
failure means an unrecoverable error or major reset.
A lock-free approach always allows progress of other threads, whereas locks can cause delays if the lock owner is preempted. However,this issue is a foundational aspect of preemptive concurrency. And there are ways to mitigate this issue for locks
using scheduler-activation techniques. However, lock-free is not immune to delays. If a page is evicted containing part of the lock-based or lockfree data, there is a delay. Hence, lock free is no better than lock based if the page
fault occurs on frequently accessed shared data. Given the increasing number of processors and large amount of memory on modern computers, neither of these delays should occur often.
Lock-free approaches are reentrant, and hence, can be used in signal handlers, which are implicitly concurrent. Locking approaches cannot deal with this issue. Lock-free approaches are claimed not to have priority inversion. However, inversion can
occur because of the spinning required with atomic instructions, like CAA, as the hardware does not provide a bound for spinning threads. Hence, a low-priority thread can barge head of a high-priority thread because the low-priority thread just happens
to win the race at the CAA instruction. Essentially,
priority inversion is a foundational aspect of preemptive concurrency and can only be mitigated.
The conclusion is that for unmanaged programming language (i.e., no garbage collection), using classical locks is simple, efficient, general, and causes issues only when the problem scales to multiple locks. For managed programming-languages, lock-free
data-structures are easier to implement, but only handle a specific set of problems, and the programmer must accept other idiosyncrasies, like pauses in
execution for garbage collection.
Thank you,
Amine Moulay Ramdane.
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