RFR: JDK-8301749: Tracking malloc pooled memory size [v3]

[Thomas Stuefe]

On Thu, 9 Feb 2023 10:38:34 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.
>
> Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Small nits

src/hotspot/os/linux/mallocInfoDcmd.cpp line 41:

> 39:   ALLOW_C_FUNCTION(::open_memstream, FILE* stream = ::open_memstream(&buf, &size);)
> 40:   if (stream == nullptr) {
> 41:     _output->print("Error: Could not call malloc_info(3)\n");

Why not print_cr() ?

src/hotspot/os/linux/mallocInfoDcmd.cpp line 48:

> 46:   if (err == 0) {
> 47:     ALLOW_C_FUNCTION(::fflush, fflush(stream);)
> 48:     _output->print_raw(buf);

Append newline (`_output->cr()`)

src/hotspot/os/linux/mallocInfoDcmd.cpp line 50:

> 48:     _output->print_raw(buf);
> 49:   } else if (err == -1) {
> 50:     _output->print("Error: %s\n", os::strerror(errno));

print_cr() ?

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