Why have i invented scalable RWLocks and scalable Locks ?

[amin...@gmail.com]

Hello,


Why have i invented scalable RWLocks and scalable Locks ?

Because there is a disadvantage with Transactional memory and
here it is:

About Hardware Transactional Memory:

"As someone who has used TSX to optimize synchronization primitives, you can expect to see a ~15-20% performance increase, if (big if) your program is heavy on disjoint data access, i.e. a lock is needed for correctness, but conflicts are rare in practice. If you have a lot of threads frequently writing the same cache lines, you are probably going to see worse performance with TSX as opposed to traditional locking. It helps to think about TSX as transparently performing optimistic concurrency control, which is actually pretty much how it is implemented under the hood."

Read more here:

https://news.ycombinator.com/item?id=8169697


So as you are noticing, HTM (hardware transactional memory) and TM can not replace locks when doing IO and when we have a highly contended critical section.


Read the rest:


I have just read the following article that appeared in C/C++ Users Journal, 23(3), March 2005

The Trouble With Locks

http://gotw.ca/publications/mill36.htm


And here is my thoughts about how to avoid deadlocks and race conditions
in lock-based systems:

https://community.idera.com/developer-tools/general-development/f/getit-and-third-party/71464/about-turing-completeness-and-parallel-programming

Also i don't agree with him about composability of lock-based systems,
read the following to understand:

"About composability of lock-based systems now:

Design your systems to be composable. Among the more galling claims of the detractors of lock-based systems is the notion that they are somehow uncomposable:

“Locks and condition variables do not support modular programming,” reads one typically brazen claim, “building large programs by gluing together smaller programs[:] locks make this impossible.”9 The claim, of course, is incorrect. For evidence one need only point at the composition of lock-based systems such as databases and operating systems into larger systems that remain entirely unaware of lower-level locking.

There are two ways to make lock-based systems completely composable, and each has its own place. First (and most obviously), one can make locking entirely internal to the subsystem. For example, in concurrent operating systems, control never returns to user level with in-kernel locks held; the locks used to implement the system itself are entirely behind the system call interface that constitutes the interface to the system. More generally, this model can work whenever a crisp interface exists between software components: as long as control flow is never returned to the caller with locks held, the subsystem will remain composable.

Second (and perhaps counterintuitively), one can achieve concurrency and
composability by having no locks whatsoever. In this case, there must be
no global subsystem state—subsystem state must be captured in per-instance state, and it must be up to consumers of the subsystem to assure that they do not access their instance in parallel. By leaving locking up to the client of the subsystem, the subsystem itself can be used concurrently by different subsystems and in different contexts. A concrete example of this is the AVL tree implementation used extensively in the Solaris kernel. As with any balanced binary tree, the implementation is sufficiently complex to merit componentization, but by not having any global state, the implementation may be used concurrently by disjoint subsystems—the only constraint is that manipulation of a single AVL tree instance must be serialized."

Read more here:

https://queue.acm.org/detail.cfm?id=1454462



Thank you,
Amine Moulat Ramdane.