RFR: 8281946: VM.native_memory should report size of shareable memory [v3]

[Ioi Lam]

On Mon, 5 Dec 2022 20:02:50 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Matias Saavedra Silva has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Comments and suggestions
>
> Changes requested by iklam (Reviewer).

> > The choice to use readonly over shareable was another suggestion by @iklam. Here is some rationale: "Other mmaped pages can be shareable if they are writable but has not been modified by the JVM process (ie copy-on-write pages)"
> 
> I still don't get it... @iklam ?
> 
> Writeable and shareable are orthogonal properties. You can have every permutation. And in the JDK we have:
> 
> * RO + shared: CDS ro segments, text segments, ...
> * RW + shared: things like perfmem at least, but possibly other things too.
> * RO + private: guard pages,  ...
> * RW + private: normal memory.
> 
> Here, we want to tell the user if something is shared: if the memory footprint is shouldered by several processes or by this process alone. The user does not really care about writeability, that's incidental. Therefore I think we should name it sharable.

I agree that the user might be more interested in shareability than writeability. However, that's not something that we can accurately report.

Regarding the ` Shared class space` part of the output, it covers only the `MetaspaceShared::ro` and `MetaspaceShared::rw` sections. Part of the `rw` sections may be shareable across processes because they are not written into during runtime. However, we cannot tell which pages are shared (unless we go through the `/proc/self/smaps` file, which would be too complicated for this RFE **)

If we report only the `ro` section as `shareable`, I think that's not accurate.

I see your point that adding `readonly` to these two lines is problematic, as we don't include the `PROT_READ` only mmap pages that are used for stackguards, for example.


Total: reserved=33890498670, committed=2360907886, readonly=8315472
       mmap:   reserved=33815339008, committed=2285748224, readonly=8315472


So for the output to be accurate, I think we should only change this one line, and leave the above two lines unchanged.


-        Shared class space (reserved=16777216, committed=13545472, readonly=8315472)


** Perhaps in a future RFE, we could go through the smaps file and report something like this for all the `mmap` lines:


       mmap:   reserved=33815339008, committed=2285748224, shared=12345

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PR: https://git.openjdk.org/jdk/pull/11401