Don’t Touch These Settings!

As briefly introduced in Garbage Collection Primer, the JVM uses a garbage collector to free unused memory. This tip is really an extension of the last tip, but deserves its own section for emphasis:

Do not change the default garbage collector!

The default GC for Elasticsearch is Concurrent-Mark and Sweep (CMS). This GC runs concurrently with the execution of the application so that it can minimize pauses. It does, however, have two stop-the-world phases. It also has trouble collecting large heaps.

Despite these downsides, it is currently the best GC for low-latency server software like Elasticsearch. The official recommendation is to use CMS.

There is a newer GC called the Garbage First GC (G1GC). This newer GC is designed to minimize pausing even more than CMS, and operate on large heaps. It works by dividing the heap into regions and predicting which regions contain the most reclaimable space. By collecting those regions first (garbage first), it can minimize pauses and operate on very large heaps.

Sounds great! Unfortunately, G1GC is still new, and fresh bugs are found routinely. These bugs are usually of the segfault variety, and will cause hard crashes. The Lucene test suite is brutal on GC algorithms, and it seems that G1GC hasn’t had the kinks worked out yet.

We would like to recommend G1GC someday, but for now, it is simply not stable enough to meet the demands of Elasticsearch and Lucene.

Everyone loves to tweak threadpools. For whatever reason, it seems people cannot resist increasing thread counts. Indexing a lot? More threads! Searching a lot? More threads! Node idling 95% of the time? More threads!

The default threadpool settings in Elasticsearch are very sensible. For all threadpools (except search) the threadcount is set to the number of CPU cores. If you have eight cores, you can be running only eight threads simultaneously. It makes sense to assign only eight threads to any particular threadpool.

Search gets a larger threadpool, and is configured to # cores * 3.

You might argue that some threads can block (such as on a disk I/O operation), which is why you need more threads. This is not a problem in Elasticsearch: much of the disk I/O is handled by threads managed by Lucene, not Elasticsearch.

Furthermore, threadpools cooperate by passing work between each other. You don’t need to worry about a networking thread blocking because it is waiting on a disk write. The networking thread will have long since handed off that work unit to another threadpool and gotten back to networking.

Finally, the compute capacity of your process is finite. Having more threads just forces the processor to switch thread contexts. A processor can run only one thread at a time, so when it needs to switch to a different thread, it stores the current state (registers, and so forth) and loads another thread. If you are lucky, the switch will happen on the same core. If you are unlucky, the switch may migrate to a different core and require transport on an inter-core communication bus.

This context switching eats up cycles simply by doing administrative housekeeping; estimates can peg it as high as 30μs on modern CPUs. So unless the thread will be blocked for longer than 30μs, it is highly likely that that time would have been better spent just processing and finishing early.

People routinely set threadpools to silly values. On eight core machines, we have run across configs with 60, 100, or even 1000 threads. These settings will simply thrash the CPU more than getting real work done.

So. Next time you want to tweak a threadpool, please don’t. And if you absolutely cannot resist, please keep your core count in mind and perhaps set the count to double. More than that is just a waste.