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ehcacheehcache-3
Daniel
Daniel

Reputation: 3370

Disk persistent cache in ehcache 3.4 is using (leaking?) direct memory

I am running a web application that makes use of Ehcache 3.4.0. I have a cache configuration that defines a simple default of 1000 in-memory objects:

<cache-template name="default">
    <key-type>java.lang.Object</key-type>
    <value-type>java.lang.Object</value-type>
    <heap unit="entries">1000</heap>
</cache-template>

I then have some disk-based caches that use this default template, but override all values (generated programmatically, so that's why they even use the default template at all) like so:

<cache alias='runViewCache' uses-template='default'>
    <key-type>java.lang.String</key-type>
    <value-type>java.lang.String</value-type>
    <resources>
        <heap unit='entries'>1</heap>
        <disk unit='GB' persistent='true'>1</disk>
    </resources>
</cache>

As data is written into my disk-based cache, direct/off-heap memory is used by the JVM, and never freed. Even clearing the cache does not free the memory. The memory used is directly related (nearly byte-for-byte as far as I can tell) to the data written to the disk-based cache.

The authoritative tier for this cache is an instance of org.ehcache.impl.internal.store.disk.OffHeapDiskStore.

This appears to be a memory leak (memory is consumed and never freed) but I am by no means an expert at configuring ehcache. Can anyone suggest a configuration change that will cause my disk tier to NOT use off-heap memory? Or, is there something else that I am just completely misunderstanding that someone else can point out?

Thank you!

Upvotes: 3

Views: 1220

Answers (1)

Eduard Wirch
Eduard Wirch

Reputation: 9922

How do you measure "used"?

TL;DR No, disk tier does not waste RAM.

As of v3.0.0 Ehcache uses memory mapped files for disk persistence:

Replacement of the port of Ehcache 2.x open source disk store by one that leverages the offheap library and memory mapped files.

This means, Ehcache uses in-memory address space to access files on disk. This does consume 0 bytes of your RAM. (At least directly. As @louis-jacomet already stated, the OS can decide to cache parts of the files in RAM.)

When you're running on Linux you should compare the VIRT and RES values of your process. VIRT is the amount of virtual bytes used by the process. RES is the amount of real RAM (RESident) bytes used by the process. VIRT should increase, while disk store cache is populated, but RES should remain pretty stable.

Upvotes: 2

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