We’ve been doing a ton of work with ElasticSearch. Not long ago, we had a few situations where ElasticSearch would “eat” all the JVM heap memory we give it. It was so hungry, we could not feed it enough memory to keep it happy. It was insatiable. After some troubleshooting and looking at SPM for ElasticSearch (btw. we released a new version of the SPM agent earlier this week, so if you don’t have it, go grab agent v1.5.0) we figured out the cause - ElasticSearch default field cache setting was not quite right for our deployment. In this post we’ll share our experience on this topic, explain why this was happening and how to minimize the negative effect of large field caches.
ElasticSearch Cache Types
There are two types of caches in ElasticSearch whose behaviors you can control. The first cache is the filter cache. This cache is responsible for caching results of filters used in your queries. This is very handy, because after a filter is run once, ElasticSearch will subsequently use values stored in the filter cache and thus save precious disk I/O operations and by avoiding disk I/O speed up query execution. There are two main implementations of filter cache in ElasticSearch:
- node filter cache (default)
- index filter cache
The node filter cache is an LRU cache, which means that the least recently used items will be evicted when the filter cache is full. Its size can be limited to be either a percentage of the total memory allocated to the Java process or by specifying the exact amount of memory. The second type of filter cache is the index filter cache. It is not recommended for use because you can’t predict (in most cases) how much memory it will use, since that depends on which shards are located on which node. In addition to that, you can’t control the amount of memory used by index filter cache, you can only set its expiration time and maximum amount of entries in that cache.
The second type of cache in ElasticSearch is field data cache. This cache is used for sorting and faceting in most cases. It loads all values from the field you sort or facet on and then provides calculations on the basis of loaded values. You can imagine that the cost of building such a cache for a large amount of data might be very high. And it is. Apart from the type (which can be either resident or soft) you can control two additional parameters of field data cache – the maximum amount of entries in it and its expiration time.
The default implementation for the filter cache is the index filter cache, with its size set to the maximum of 20% of the memory allocated to the Java process. As you can imagine there is nothing to worry about – if the cache fills up appropriate cache entries will get evicted. You can then consider adding more RAM to make index filter cache bigger or you must live with evictions. That’s perfectly acceptable.
On the other hand we have the default settings for ElasticSearch field data cache – it is a resident cache with unlimited size. Yes, unlimited. The cost of rebuilding this cache is very high and thus you must know how much memory it can use – you must control your queries and watch what you sort on and on which fields you do the faceting.
What Happens When You Don’t Control Your Cache Size ?
This is what can happen when you don’t control your field data cache size:
As you can see on the above chart field data cache jumped to more than 58 GB, which is enormous. Yes, we got OutOfMemory exception during that time.
What CanYou Do ?
There are actually three thing you can do to make your field data cache use less memory:
Control its Size and Expiration Time
When using the default, resident field data cache type, you can set its size and expiration time. However, please remember, that there are situations when you need the field data cache to hold values for that particular field you are sorting or faceting on. In order to change field data cache size, you specify the following property:
It specifies the maximum size entries in that cache per Lucene segment. It doesn't limit the amount of memory field data cache can use, so you have to do some testing to ensure queries you are using won't result in OutOfMemory exception.
The other property you can set is the expiration time. It defaults to -1 which says that the cache will not be expired by default. In order to change that, you must set the following property:
So if, for example, you would like to have a maximum of 50k entries of field data cache per segment and if you would like to have those entries expiredafter 10 minutes, you would set the following property values in ElasticSearch configuration file:
Change its Type
The other thing you can do is change field data cache type from the default resident to soft. Why does that matter? ElasticSearch uses Google Guava libraries to implement its cache. The soft type wraps cache values in soft references, which means that whenever memory is needed garbage collector will clear those references even when they are used. This means that when you start hitting heap memory limit, the JVM wont throw OutOfMemory exception, but will instead release those soft references with the use of garbage collector. More about soft references can be found at:
So in order to change the default field data cache type to soft you should add the following property to ElasticSearch configuration file:
Change Your Data
The last thing you can do is the operation that requires much more effort than only changing ElasticSearch configuration - you may want to change your data. Look at your index structure, look at your queries and think. Maybe you can lowercase some string data and this way reduce the number of unique values in the field? Maybe you don't need your dates be precise down to a second, maybe it can be minute or even an hour? Of course, when doing some faceting operations you can set the granularity, but the data will still be loaded into memory. So if there are parts of your data that can be changed in a way that will result in lower memory consumption, you should consider it. As a matter of fact, that is exactly what we did!
Caches After Some Changes
After we made some changes in our ElasticSearch configuration/deployment, this is what the field data cache usage looked like:
As you can see, the cache dropped from 58 GB down to 37 GB. Impressive drop! After these changes we stopped running into OutOfMemory exception problems.
You have to remember that the default settings for field data cache in ElasticSearch may not be appropriate for you. Of course, that may not be the case in your deployment. You may not need sorting, apart from the default based on Lucene scoring and you may not need faceting on fields with many unique terms. If that's the case for you, don't worry about field data cache. If you have enough memory for holding the fields data for your facets and sorting then you also don't need to change anything regarding the cache setup from the default ElasticSearch configuration. What you need to remember is to monitor your JVM heap memory usage and cache statistics, so you know what is happening in your cluster and react before things get worse.
One More Thing
The charts you see in the post are taken from SPM (Scalable Performance Monitoring) for ElasticSearch. SPM is currently free and, as you can see, we use it extensively in our client engagements. If you give it a try, please let us know what you think and what else you would like to see in it.
-- @sematext (Like working with ElasticSearch? We're hiring!)