`ObjectMapper` default heap consumption increased significantly from 2.13.x to 2.14.0

Hello,

I’m currently looking at heap consumption when using native image with a Spring Boot 3 application. Spring Boot 3.0 SNAPSHOT uses Jackson 2.14.0-rc3. I noticed that with the upgrade from 2.13.4.2 to 2.14.0 the application uses more heap memory, which i tracked down to this PR. It changed the cache implementation from ConcurrentHashMap to PrivateMaxEntriesMap, which in turn preallocates a lot of AtomicReference (they appear to be all empty).

I created a reproducer here - it creates an empty objectmapper and then dumps the heap. It has more information and screenshots in the README.

When using Jackson 2.13.4.2 it uses 567 B for the LRUmap, with Jackson 2.14.0 it uses 507320 B for the LRU map. The bad thing is that most of these caches are per ObjectMapper - create more of them and it uses even more heap.

As we’re trying to optimize the memory consumption of native image applications, this is quite a bummer and we hope you can help us out here.

Thanks!

[cowtowncoder]

Ta-dah! I did it. 2.14.1 release mostly done, last couple of artifacts on their way to Maven Central (Scala module may take bit longer). Please LMK if you find issues.

[ben-manes]

@pjfanning you forgot to account for the AtomicReference instances themselves. That ranges from 16-32 bytes depending on the jvm optimizations (compressed references, compressed class pointers, byte alignment). In that case you might expect to see it as 16 * 128 (4 + 32) = 73,728 or ~300kb if each mapper uses 4 caches. Using Java Object Layout,

# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

... (many variations) ...

***** Hotspot Layout Simulation (JDK 15, 64-bit model, compressed class pointers, 16-byte aligned)
java.util.concurrent.atomic.AtomicReference object internals:
OFF  SZ               TYPE DESCRIPTION               VALUE
  0   8                    (object header: mark)     N/A
  8   4                    (object header: class)    N/A
 12   4                    (alignment/padding gap)
 16   8   java.lang.Object AtomicReference.value     N/A
 24   8                    (object alignment gap)
Instance size: 32 bytes
Space losses: 4 bytes internal + 8 bytes external = 12 bytes total