RFR: JDK-8301749: Tracking malloc pooled memory size

[David Holmes]

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.

General idea is fine - thanks for implementing.

There is unnecessary ifdef usage, and also a query about the existence check, below.

This needs a jcmd documentation update - closed MR to update ` closed/src/jdk.jcmd/share/man/jcmd.md` and then include the updated jcmd.1 manpage file here.

src/hotspot/os/linux/mallocInfoDcmd.cpp line 35:

> 33: 
> 34: void MallocInfoDcmd::execute(DCmdSource source, TRAPS) {
> 35: #ifdef LINUX

This is a linux-only file. ???

src/hotspot/os/linux/os_linux.cpp line 224:

> 222: 
> 223: int os::Linux::malloc_info(FILE* stream) {
> 224:   if (::malloc_info) {

This looks strange as an existence check for a library function. At build time what does this get compiled into such that it runtime it can determine whether `malloc_info` exists or not ??

(also style rules say no implicit booleans so check against nullptr or 0).

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Changes requested by dholmes (Reviewer).

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