• Lock Versus Lock-Free..

    From Wisdom90@21:1/5 to All on Thu Jun 11 20:45:16 2020

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    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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