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Under extreme circumstances, one o r more threads could b e locked out (Le, livelocked)
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T e 1 2 3 4 5 6 7 8 t1 c t2
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s_c o u n t e r ( e ) c s_c o u n t e r ( 1 ) Comp a r e E x c hange ( e , 1 ) ( s u c c es s )
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CompareE x c h a nge ( e , 1 ) ( f a i l ! ) c s_counter ( 1 )
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c s_c ounter ( 1 ) CompareExchange ( l , 2 ) ( s u c c es s ) Compa reE x c h a nge ( l , 2 ) ( fa i l ! ) c s_c o u n t e r ( 2 )
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In this example, t 1 keeps getting beat out b y t2, leading to i t retrying over and over again While it's unlikely such extreme examples would arise, the example does illustrate the point This is an example that only results in a single thread being livelocked One can easily imagine situations where two threads are cooperating and both end up backing off voluntarily to retry some operation Imagine if we implemented the simultaneous lock acquisition code earlier by trying to acquire locks in the order supplied If one thread tried to acquire lock A and then B, while another tried to acquire lock B and then A, deadlock could occur To cope, we might use timeouts and "roll back" successful acquisi tions upon contention; we then spin briefly and try again If all threads par ticipate in this scheme, they may interfere with one another, back off, retry, interfere yet again, and so on, indefinitely In both cases, threads use up a lot of processor time without making any true forward progress This can result in hard to explain delays in process ing and drops in throughput Livelock is just a fact of life Algorithms deep down in the Windows OS and in the CLR suffer from these kinds of issues They rely on the fact that, probabilistic speaking, indefinite livelock will not happen There are too many subtle timing issues involved in order to produce most indefinite livelocks: cache misses, context switches, background services, foreground applications, disk and memory access latencies, and the like That said, randomized backoff is a popular technique that decreases the chances even further of a thread being indefinitely delayed This is a
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technique we explore in 1 4, Performance and Scalability, when looking at spin wait algorithms The idea is that, upon failure and before retrying an operation, a thread spins for a random amount of time More over, for each failed attempt at an operation, the amount of spin delay used will be increased Provided that all threads in the system cooperate by using the same backoff logic, the chance of having many threads enter a true livelock situation is rare
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Lock Convoys Lock convoys are situations where the arrival rate for a lock is high com pared to the release rate Convoys can have a dramatic impact on scalabil ity, leading to threads being backed up waiting for a lock (or event) and, in many cases, a substantial drop in throughput A convoy is most often due to a fundamental architectural problem in a system, but can also be exac erbated by the implementation of synchronization primitives as well as runtime and OS features Two conditions are typically involved when a convoy occurs
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The arrival rate for some lock is high In other words, a nontrivial amount of the program's execution happens under the protection of a particular lock The hold time for that same lock is also high In other words, once a thread acquires the lock, it doesn't release it for some a lengthy period of time
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Some simple mathematics can be used to describe the problem Imag ine the arrival rate for a lock is 1 thread / l O,OOO cycles If the average lock hold time is any higher than 1 0,000 cycles, a convoy will ensue, and threads will arrive more frequently than locks are granted Imagine the average hold time is also exactly 1 0,000 cycles The system will be perfectly bal anced in a sense and in theory, but in practice, random delays due to cache misses and page faults can throw this balance out of whack without notice One thread holding the lock for 1 5,000 cycles is enough to cause the wait queue to grow Unless a subsequent thread holds the lock for 5,000 or less cycles to offset this balance (or the arrival rate slows), we will not recover
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