android/guava/src/com/google/common/cache/CacheBuilder.java
public CacheBuilder<K, V> concurrencyLevel(int concurrencyLevel) {
    checkState(
        this.concurrencyLevel == UNSET_INT,
        "concurrency level was already set to %s",
        this.concurrencyLevel);
    checkArgument(concurrencyLevel > 0);
    this.concurrencyLevel = concurrencyLevel;
    return this;
  }
public CacheBuilder<K, V> concurrencyLevel(int concurrencyLevel) {
    checkState(
        this.concurrencyLevel == UNSET_INT,
        "concurrency level was already set to %s",
        this.concurrencyLevel);
    checkArgument(concurrencyLevel > 0);
    this.concurrencyLevel = concurrencyLevel;
    return this;
  }
Guides the allowed concurrency among update operations. Used as a hint for internal sizing. The
table is internally partitioned to try to permit the indicated number of concurrent updates
without contention. Because assignment of entries to these partitions is not necessarily
uniform, the actual concurrency observed may 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 permits only one thread to modify the cache
at a time, but since read operations and cache loading computations can proceed concurrently,
this still yields higher concurrency than full synchronization.

<p>Defaults to 4. <b>Note:</b>The default may change in the future. If you care about this
value, you should always choose it explicitly.

<p>The current implementation uses the concurrency level to create a fixed number of hashtable
segments, each governed by its own write lock. The segment lock is taken once for each explicit
write, and twice for each cache loading computation (once prior to loading the new value, and
once after loading completes). Much internal cache management is performed at the segment
granularity. For example, access queues and write queues are kept per segment when they are
required by the selected eviction algorithm. As such, when writing unit tests it is not
uncommon to specify {@code concurrencyLevel(1)} in order to achieve more deterministic eviction
behavior.

<p>Note that future implementations may abandon segment locking in favor of more advanced
concurrency controls.

@return this {@code CacheBuilder} instance (for chaining)
@throws IllegalArgumentException if {@code concurrencyLevel} is nonpositive
@throws IllegalStateException if a concurrency level was already set