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Design

Sharing Is the Root of All Contention


Visible Costs (A1, A2, A3)

Consider the following code which takes locks for synchronization to control access to a shared object x:


// Example 1: Good, if a little boring;
// plain-jane synchronized code using locks

// Thread 1
mutX.lock();           // A1, A2: potentially blocking call
Modify( x, thisWay );	
mutX.unlock();         // A2: more expense for synchronization

// Thread 2
mutX.lock();           // A1, A2: potentially blocking call
Modify( x, thatWay );  // ok, no race
mutX.unlock();         // A2: more expense for synchronization

// Thread 3
mutX.lock();           // A1, A2: potentially blocking call
Modify( x, anotherWay ); // ok, no race
mutX.unlock();         // A2: more expense for synchronization

The obvious waiting costs (type A1) are clear: You can read them right in the source code where it says mutX.lock(). If these threads try to run concurrently, the locks force them to wait for each other; only one thread may execute in the critical section at a time, and the use of x is properly serialized.

The A1 waiting characteristics may vary. For example, if mutX is a reader/writer mutex, that will tend to improve concurrency among readers, and readers' lock acquisitions will succeed more often and so be cheaper on average; but acquiring a write lock will tend to block more often as writers are more likely to have to wait for readers to clear. We could also rewrite the synchronization in terms of atomic variables and compare-and-swap (CAS) operations, perhaps with a CAS loop that optimistically assumes no contention and goes back and tries again if contention happens, which is common in all but the best lock-free algorithms (lock-free and obstruction-free; only wait-free algorithms avoid A1 entirely). [2]

We also incur the performance tax of executing the synchronization operations themselves (A2). Some of these are probably obvious; for example, taking a lock on a contended mutex clearly requires spinning or performing a context switch which can be expensive. But even taking an uncontended lock on an available mutex isn't free, because it requires a synchronized memory operation such as a compare-and-swap (CAS). And, perhaps surprisingly, so does releasing a lock with mutX.unlock(), because releasing a lock requires writing a value that signifies the mutex is now free, and that write has to be synchronized and propagated across the system to other caches and cores. (We'll have more to say about memory synchronization when we get to cases C1-C3.)

Even the best wait-free algorithms can't avoid paying this toll, because wait-free algorithms are all about avoiding the waiting costs (A1); they still rely on synchronized atomic memory operations and so must pay the overhead taxes (A2). On many mainstream systems, the write to unlock a mutex, or to write or CAS an atomic variable in lock-free code, is significantly more expensive than a normal memory write for reasons we'll consider in a moment, even when there are no other contending threads on other processors that will ever care to look at the new value.

Finally, some algorithms may actually have to redo their work (A3), particularly if they perform optimistic execution. Lock-free code to change a data structure will frequently do its work assuming that no other writers are present and that its final atomic "commit" will succeed, and perform the commit using a CAS operation that lets it confirm whether the key variable still has its original expected value; if not, then the commit attempt failed and the code will need to loop to try again and attempt a CAS commit again (hence, the term "CAS loop") until it succeeds. Each time the code has to retry such a loop, it may need to partially or entirely throw away the work it did the previous time through. The more contention there is, the more often it will have to re-execute. On top of losing the work, there is often an extra overhead if abandoning the work requires that we do some cleanup (e.g., deallocation or destruction/disposal of scratch resources) or perform compensating actions to "undo" or "roll back" the work.

Besides shared memory operations, other kinds of shared resources cause similar problems. For example, if two processes try to open a file for exclusive access with a blocking call, one must wait (A1), for the same reason one child must wait for another to finish using a shared green crayon. Opening a file for exclusive use is also usually a more expensive operation than opening it for read-only use because it incurs more coordination overhead (A2); it's the same kind of overhead as when two coffee customers are waiting for the cream, and after waiting for the previous customer to finish they have to make eye contact and maybe even talk to decide which of the waiters gets to go next.

In all these cases, the key point is that the potential blocking or overhead or retrying is clearly visible in the source code. You can point to the lines that manipulate a mutex or an atomic variable or perform CAS loops and see the costs. Unfortunately, life isn't always that sweet.

Invisible Costs in Software (B1, B2, B3)

Column B lists examples of some costs of sharing that still occur in software, but typically aren't visible by reading your own source code.

Two pieces of code may have to wait for each other because of synchronization being done under the covers by low-level libraries or by the operating system (B1). This can arise because they share a resource (not just memory): For example, two threads may try to write to the console, or append to the same file, using a synchronized I/O mode; if they try to execute their conflicting actions at the same time, the I/O library will serialize them so that one will block and wait until the other is done before proceeding. As another example, if two threads are running on the same CPU core, the operating system will let them share the core and have the illusion of executing simultaneously by making them take turns, interleaving their execution so that each periodically is preempted and waits for the other to get and use its next quantum.

These kinds of invisible sharing can also impose invisible synchronization overhead costs (B2). In the case of two threads sharing a CPU core, in addition to waiting they incur the overhead of context switches as the operating system has to save and restore the registers and other essential state of each thread; thanks to that tax, the total CPU time the threads receive adds up to less than 100 percent of the processor's capacity.

While we're speaking of B2, I have some mixed news for fans of reader/writer mutexes and reference counting. A reader/writer mutex is a common tool to achieve better concurrency for a "read-mostly" shared object. The idea is that, since readers dominate and won't interfere with each other, we should let them run concurrently with each other. This can work pretty well, but taking a read lock requires at least an expensive CAS on the shared reference count, and the CAS expense grows with the number of participating threads and processors. (Note that many reader/writer mutexes require much more overhead than just that because they maintain other lists, counters, and flags under the covers, so that they can block readers when a writer is waiting or active, allow priority entry, and maintain fairness and other properties.) [12]. The shorter the reader critical section and the more frequently that it's called, the more the synchronization overhead will begin to dominate. A similar problem arises with plain old reference counting, where the CAS operations to maintain the shared reference count itself can limit scalability. We'll consider the reasons why shared writes in general, and CAS in particular, are so expensive in more detail when talking about column C (see below).

Sharing also inhibits optimizations (still B2). When we share a resource, and notably when we share an object in memory, directly or indirectly we have to identify critical sections in the code to acquire and release the resource. As I described in detail in [2] and [3], compilers (and processors and caches, in C2) have to restrict the optimizations they can perform across those barriers, and that can result in a measurable penalty especially in inner loops.

Finally, for similar reasons as those we saw with A3, backoff algorithms and optimistic algorithms that are implemented under the covers inside libraries or by the operating system can result in invisible wasted work (B3).

That concludes our tour of software-related costs...but the story would be grossly incomplete without looking in detail at costs in hardware. Let's focus first once again on the important case of shared memory objects, then broaden the net again to remember that the principle is fully general.


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