RFR: JDK-8301749: Tracking malloc pooled memory size

[Robbin Ehn]

On Tue, 7 Feb 2023 15:00:14 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 wondering if we should try to warn in the case when user have preloaded a thrid party malloc that don't have malloc_info. But when we load libc that symbol will find a target.
If it's not to much trouble it would be pretty nice if we could return some output to the user indicating this.
Since there a few ways to high-jack malloc maybe it's hard to figure out and argubly if the user preloaded something he should be aware of it, but maybe not aware if it contains malloc_info or not.

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