Hash reported as retained

[cristiangreco]

MemoryProfiler will report that an object of type Hash is being retained by the following code:

require "memory_profiler"

def run
  {{:a => 1} => 2}
end

MemoryProfiler.report { run }.pretty_print

Output:

Total allocated: 696 bytes (3 objects)
Total retained:  232 bytes (1 objects)

allocated memory by gem
-----------------------------------
       696  other

allocated memory by file
-----------------------------------
       696  report.rb

allocated memory by location
-----------------------------------
       464  report.rb:5
       232  report.rb:4

allocated memory by class
-----------------------------------
       696  Hash

allocated objects by gem
-----------------------------------
         3  other

allocated objects by file
-----------------------------------
         3  report.rb

allocated objects by location
-----------------------------------
         2  report.rb:5
         1  report.rb:4

allocated objects by class
-----------------------------------
         3  Hash

retained memory by gem
-----------------------------------
       232  other

retained memory by file
-----------------------------------
       232  report.rb

retained memory by location
-----------------------------------
       232  report.rb:5

retained memory by class
-----------------------------------
       232  Hash

retained objects by gem
-----------------------------------
         1  other

retained objects by file
-----------------------------------
         1  report.rb

retained objects by location
-----------------------------------
         1  report.rb:5

retained objects by class
-----------------------------------
         1  Hash

I've actually observed similar behaviour when using an array or a custom class as hash key.

What does it mean the Hash is being retained in such cases? What do you think is preventing the object being unallocated?

[cristiangreco]

@SamSaffron any chance you can have a quick look into this? 😊

[SamSaffron]

It is legitimately being retained see:

https://gist.github.com/e58a0c9743d1927e65e6968f7c050e52

Recommend you open an issue with Ruby core here

[cristiangreco]

@SamSaffron brilliant, thanks for confirming I was not (yet) going crazy with tracking down a memory leak. Will let you know once I've forwarded this to ruby core devs.

[cristiangreco]

@SamSaffron FYI I've been suggested by ruby devs that this is not a case of memory leak: the hash is most probably not being immediately deallocated due to the GC being conservative. https://bugs.ruby-lang.org/issues/16278

It's unfortunate that object space analysis doesn't help in such cases.