RFR: JDK-8301749: Tracking malloc pooled memory size

[Poonam Bajaj]

On Tue, 7 Feb 2023 15:08:11 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> This PR implements `os::malloc_info` by calling into glibc's `malloc_info` and provides a JCmd for asking the JVM to call `malloc_info` and print out the results. The user can make an informed decision regarding heap trimming by parsing the output of the JCmd. I wanted to avoid parsing the XML inside of the JVM , as this functionality may turn out to be "good enough". A future enhancement would be for the JVM to make this informed decision by parsing the output. Another may be to present this with other data, for example in NMT.
>> 
>> @tstuefe, I think you'd be interested in this PR as it builds upon your `trim_native_heap` functionality.
>> 
>> I'm using weak symbols (as per ELF) in order to compile this for both GLibc and Musl.
>> 
>> The original bug description is this:
>> 
>>>Memory not allocated via NMT and uncommitted memory contributes to a discrepancy between NMT and RSS.
>> It can be hard to distinguish between these cases.
>> If the VM can determine the amount of pooled memory by malloc it would help with both determining if a trim_native_memory is needed, or there are some allocations happening outside of NMT.
>>>
>>>It would be good if the VM can summarize the malloc pooled memory by using e.g. malloc_info(3).
>>>
>>>We can thus more precise account for RSS value.
>
> I'm opening this PR as a suggested change. I'm not strongly attached to the idea that it must be a JCmd, or that we must simply return the info as it is. I am only strongly attached to the idea that the user should have an easy way of receiving the information that `malloc_info` offers. If anyone has any suggestions, please share them!

@jdksjolen would be good to share the output of this command. Would give an idea to the reviewers as to what information gets printed with this command.

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PR: https://git.openjdk.org/jdk/pull/12455