Allow finer grained locking in SolrCores

Access to loaded SolrCore instances is a synchronized read and write operation in SolrCores#getCoreFromAnyList. This method is touched by every request as every HTTP request is assigned the SolrCore it operates on.

Background

Under heavy load we discovered that application halts inside of Solr are becoming a serious problem in high traffic environments. Using Java Flight Recordings we discovered high accumulated applications halts on the modifyLock in SolrCores. In our case this means that we can only utilize our machines up to 25% cpu usage. With the fix applied, a utilization up to 80% is perfectly doable.

In our case this specific locking problem was masked by another locking problem in the SlowCompositeReaderWrapper. We'll submit our fix to the locking problem in the SlowCompositeReaderWrapper in a following issue.

Problem

Our Solr instances utilizes the collapse component heavily. The instances run with 32 cores and 32gb Java heap on a rather small index (4gb). The instances scale out at 50% cpu load. We take Java Flight Recorder snapshots of 60 seconds
as soon the cpu usage exceeds 50%.

During our 60s Java Flight Recorder snapshot, the ~2k Jetty acceptor threads accumulated more than 12h locking time inside SolrCores on the modifyLock instance used as synchronized lock (see screenshot). With this fix the locking access is reduced to write accesses only. We validated this using another JFR snapshot:

We ran this code for a couple of weeks in our live environment.

Solution

The synchronized modifyLock is replaced by a ReentrantReadWriteLock. This allows concurrent reads from the internal SolrCore instance list (cores) but grants exclusive access to write operations on the instance list (cores).

In Solr 9.x the cache inside the TransientSolrCoreCacheFactoryDefault adds a cache overflow handling of the size based internal cache (SOLR-15964). As soon as SolrCores are evicted from the internal cache, the cache behaviour changes from a size based cache to a reference based cache via the cache's eviction handler. SolrCore instances that are still referenced are inserted back into the cache. This means that write operations to the cache (insert SolrCore) can be issued during read operations in SolrCores. Hence these operations have only a read lock which cannot be upgraded to a write lock (dead lock).

To overcome this, we moved the cache maintenance (including the eviction handler) in TransientSolrCoreCacheFactoryDefault to a separate thread. This thread can acquire a write lock but on the other hand a separate thread will schedule a ping-pong behaviour in the eviction handler on a full cache with SolrCores still referenced. To overcome this we made the overflow behaviour transparent by adding an additional overflowCores instance. Here we add evicted but still referenced cores from the transientCores cache.

Furthermore we need to ensure that only a single transientSolrCoreCache inside TransientSolrCoreCacheFactoryDefault is created. As we now allow multiple read threads, we call the the getTransientCacheHandler() method initially holding a write lock inside the load() method. Calling the method only needs a write lock initially (for cache creation). For all other calls, a read lock is sufficient. By default, the getTransientCacheHandler() acquires a read lock. If a write is needed (e.g. for core creation), the getTransientCacheHandlerInternal() is called. This method explicitly does not use a lock in order to provide the flexibility to choose between a read-lock and a write-lock. This ensures that a single instance of transientSolrCoreCache is created.

The lock signaling between SolrCore and CoreContainer gets replaced by a Condition that is tied to the write lock.

Summary

This change allows for a finer grained access to the list of open SolrCores. The decreased blocking read access is noticeable in decreased blocking times of the Solr application (see screenshot).

This change has been composed together by Dennis Berger, Torsten Bøgh Köster and Marco Petris.