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Limiting Memory Usage

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Limiting Memory Usage

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In order for aggregations (or any operation that requires access to field values) to be fast, access to fielddata must be fast, which is why it is loaded into memory. But loading too much data into memory will cause slow garbage collections as the JVM tries to find extra space in the heap, or possibly even an OutOfMemory exception.

It may surprise you to find that Elasticsearch does not load into fielddata just the values for the documents that match your query. It loads the values for all documents in your index, even documents with a different _type!

The logic is: if you need access to documents X, Y, and Z for this query, you will probably need access to other documents in the next query. It is cheaper to load all values once, and to keep them in memory, than to have to scan the inverted index on every request.

The JVM heap is a limited resource that should be used wisely. A number of mechanisms exist to limit the impact of fielddata on heap usage. These limits are important because abuse of the heap will cause node instability (thanks to slow garbage collections) or even node death (with an OutOfMemory exception).

Choosing a Heap Size

There are two rules to apply when setting the Elasticsearch heap size, with the $ES_HEAP_SIZE environment variable:

No more than 50% of available RAM: Lucene makes good use of the filesystem caches, which are managed by the kernel. Without enough filesystem cache space, performance will suffer.

No more than 32 GB: If the heap is less than 32 GB, the JVM can use compressed pointers, which saves a lot of memory: 4 bytes per pointer instead of 8 bytes.

+ Increasing the heap from 32 GB to 34 GB would mean that you have much less memory available, because all pointers are taking double the space. Also, with bigger heaps, garbage collection becomes more costly and can result in node instability.

This limit has a direct impact on the amount of memory that can be devoted to fielddata.

Fielddata Size

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The indices.fielddata.cache.size controls how much heap space is allocated to fielddata. When you run a query that requires access to new field values, it will load the values into memory and then try to add them to fielddata. If the resulting fielddata size would exceed the specified size, other values would be evicted in order to make space.

By default, this setting is unbounded—Elasticsearch will never evict data from fielddata.

This default was chosen deliberately: fielddata is not a transient cache. It is an in-memory data structure that must be accessible for fast execution, and it is expensive to build. If you have to reload data for every request, performance is going to be awful.

A bounded size forces the data structure to evict data. We will look at when to set this value, but first a warning:

This setting is a safeguard, not a solution for insufficient memory.

If you don’t have enough memory to keep your fielddata resident in memory, Elasticsearch will constantly have to reload data from disk, and evict other data to make space. Evictions cause heavy disk I/O and generate a large amount of garbage in memory, which must be garbage collected later on.

Imagine that you are indexing logs, using a new index every day. Normally you are interested in data from only the last day or two. Although you keep older indices around, you seldom need to query them. However, with the default settings, the fielddata from the old indices is never evicted! fielddata will just keep on growing until you trip the fielddata circuit breaker (see Circuit Breaker), which will prevent you from loading any more fielddata.

At that point, you’re stuck. While you can still run queries that access fielddata from the old indices, you can’t load any new values. Instead, we should evict old values to make space for the new values.

To prevent this scenario, place an upper limit on the fielddata by adding this setting to the config/elasticsearch.yml file:

indices.fielddata.cache.size:  40%

Can be set to a percentage of the heap size, or a concrete value like 5gb

With this setting in place, the least recently used fielddata will be evicted to make space for newly loaded data.

There is another setting that you may see online: indices.fielddata.cache.expire.

We beg that you never use this setting! It will likely be deprecated in the future.

This setting tells Elasticsearch to evict values from fielddata if they are older than expire, whether the values are being used or not.

This is terrible for performance. Evictions are costly, and this effectively schedules evictions on purpose, for no real gain.

There isn’t a good reason to use this setting; we literally cannot theory-craft a hypothetically useful situation. It exists only for backward compatibility at the moment. We mention the setting in this book only since, sadly, it has been recommended in various articles on the Internet as a good performance tip.

It is not. Never use it!

Monitoring fielddata

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It is important to keep a close watch on how much memory is being used by fielddata, and whether any data is being evicted. High eviction counts can indicate a serious resource issue and a reason for poor performance.

Fielddata usage can be monitored:

per-index using the indices-stats API:
```
GET /_stats/fielddata?fields=*
```

per-node using the nodes-stats API:

GET /_nodes/stats/indices/fielddata?fields=*

Or even per-index per-node:

GET /_nodes/stats/indices/fielddata?level=indices&fields=*

By setting ?fields=*, the memory usage is broken down for each field.

Circuit Breaker

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An astute reader might have noticed a problem with the fielddata size settings. fielddata size is checked after the data is loaded. What happens if a query arrives that tries to load more into fielddata than available memory? The answer is ugly: you would get an OutOfMemoryException.

Elasticsearch includes a fielddata circuit breaker that is designed to deal with this situation. The circuit breaker estimates the memory requirements of a query by introspecting the fields involved (their type, cardinality, size, and so forth). It then checks to see whether loading the required fielddata would push the total fielddata size over the configured percentage of the heap.

If the estimated query size is larger than the limit, the circuit breaker is tripped and the query will be aborted and return an exception. This happens before data is loaded, which means that you won’t hit an OutOfMemoryException.

Available Circuit Breakers

Elasticsearch has a family of circuit breakers, all of which work to ensure that memory limits are not exceeded:

indices.breaker.fielddata.limit: The fielddata circuit breaker limits the size of fielddata to 60% of the heap, by default.
indices.breaker.request.limit: The request circuit breaker estimates the size of structures required to complete other parts of a request, such as creating aggregation buckets, and limits them to 40% of the heap, by default.
indices.breaker.total.limit: The total circuit breaker wraps the request and fielddata circuit breakers to ensure that the combination of the two doesn’t use more than 70% of the heap by default.

The circuit breaker limits can be specified in the config/elasticsearch.yml file, or can be updated dynamically on a live cluster:

PUT /_cluster/settings
{
  "persistent" : {
    "indices.breaker.fielddata.limit" : "40%" 
  }
}

The limit is a percentage of the heap.

It is best to configure the circuit breaker with a relatively conservative value. Remember that fielddata needs to share the heap with the request circuit breaker, the indexing memory buffer, the filter cache, Lucene data structures for open indices, and various other transient data structures. For this reason, it defaults to a fairly conservative 60%. Overly optimistic settings can cause potential OOM exceptions, which will take down an entire node.

On the other hand, an overly conservative value will simply return a query exception that can be handled by your application. An exception is better than a crash. These exceptions should also encourage you to reassess your query: why does a single query need more than 60% of the heap?

In Fielddata Size, we spoke about adding a limit to the size of fielddata, to ensure that old unused fielddata can be evicted. The relationship between indices.fielddata.cache.size and indices.breaker.fielddata.limit is an important one. If the circuit-breaker limit is lower than the cache size, no data will ever be evicted. In order for it to work properly, the circuit breaker limit must be higher than the cache size.

It is important to note that the circuit breaker compares estimated query size against the total heap size, not against the actual amount of heap memory used. This is done for a variety of technical reasons (for example, the heap may look full but is actually just garbage waiting to be collected, which is hard to estimate properly). But as the end user, this means the setting needs to be conservative, since it is comparing against total heap, not free heap.

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