CODEWITHSUNDEEP
javadata-structureshashtablejava.util.concurrent
Geek
Geek

Reputation: 27203

What guarantee does concurrencyLevel argument of ConcurrentHashMap give us?

From the Javadocs of ConcurrentHashMap :

The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument (default 16), which is used as a hint for internal sizing.

I do not understand the part that says "which is used as a hint for internal sizing." . What does this mean ? What is the best practice for setting this value and what guarantee does it give us ?

Upvotes: 2

Views: 150

Answers (2)

kodstark
kodstark

Reputation: 490

Concurrency level is around equal how many operations on map can be invoked concurrently without using internal locking mechanism. As maat b is saying that ConcurrentHashMap will have N internal hashtables and thus operations which are working on different hashtables doesn't require additional locking - otherwise if operations are working on the same internal hashtable then ConcurrenyHashMap uses additional internal locking on them.

Upvotes: 0

matt b
matt b

Reputation: 139931

Take a look at the very next sentences in the Javadoc:

The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash table is a relatively slow operation, so, when possible, it is a good idea to provide estimates of expected table sizes in constructors.

So in other words, a concurencyLevel of 16 means that the ConcurrentHashMap internally creates 16 separate hashtables in which to store data. Operations that modify data in one hashtable do not require locking the other hashtables, which allows somewhat-concurrent access to the overall Map.

You might want to try reading the source of ConcurrentHashMap.

Upvotes: 3

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