Is Guava Cache's read lock free
caching, guava, java
Solution
You can take a look at the code yourself. Most of the relevant stuff is in `LocalCache`.
Anyway, from my understanding (I'm not an expert on the cache stuff), reads (on a non-`LoadingCache`) should generally be lock-free (on a `LoadingCache` it depends on whether they need to load a value, in which case the read becomes a write too). In some code paths `tryLock()` is used to acquire a lock, but since it's `tryLock()` no reading threads should block if the lock is held I don't think.
Edit:
From comments in `Segment`:
Segments maintain a table of entry lists that are ALWAYS kept in a consistent state, so can be read without locking.
(`concurrencyLevel(1)` currently means 1 `Segment`.)
Problem
I am using Guava's Cache and have set `concurrencyLevel` to `1` so the maximum capacity is utilized w.r.t. eviction is concerned. Following is my code: ``` CacheBuilder.newBuilder() .maximumWeight(maxWeight) .concurrencyLevel(1) .expireAfterWrite(expireDuration, TimeUnit.MINUTES) .removalListener(new RemovalListener<K, V>() { @Override public void onRemoval(RemovalNotification<K, V> removalNotification) { log.warn(removalMsg + removalNotification.getKey()); } }) .weigher(weigher) .build(); ``` Documentation of concurrencyLevel says 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... And hence I was assuming that the `concurrencyLevel` plays a role only for `update operations` and the `READs are Lock Free` even with `concurrencyLevel(1)`. Is my assumption correct ?? EDIT: I looked at the Guava Caching codeand it looks like my assumption that `READs are lock free` is correct. In case of `get(K, Callable)` initially it tries to get the value without lock and if the entry for the specified key is either null or has expired only then it goes for a locked Get or Load. Please Correct me if I am wrong here.