NMT and adjacent reserved regions

[Zhengyu Gu]

>     Hi Thomas,
> 
>     IIRC, we do intend to collapse neighboring reserved regions, iff
> 
>     1) same memory type
>     2) same allocation site
> 
> 
> Hm okay. Something to keep in mind when splitting memory regions. My 
> original plan was to split the underlying NMT region into two, with both 
> keeping the original stack + flags. I guess I have to avoid doing that.
> 
> Thinking about this I am not sure collapsing is worth the trouble 
> though. Lets say I were to reserve two regions with mtInternal, but the 
> reservation calls are hidden in deep call stacks, the call stacks would 
> be identical and look like the same allocation site to NMT, right? So it 
> could happen that two reservations are collapsed and the caller is not 
> aware of that. So, I later could not re-assign different flags to those 
> regions.
> 
An option: not collapsing reserved regions internally, but collapsing 
them at reporting time?

-Zhengyu

> Thanks, Thomas
> 
>     Thanks,
> 
>     -Zhengyu
> 
>     On 5/14/20 7:30 AM, Thomas Stüfe wrote:
>      > Hi,
>      >
>      > I am currently working at a fix for JDK-8243535
>      > <https://bugs.openjdk.java.net/browse/JDK-8243535> (NMT does not
>     handle
>      > os::split_reserved_space()).
>      >
>      > I was looking at how NMT treats adjacent reserved regions. Do we
>     collapse
>      > them in NMT? The code here
>      >
>      >
>     http://hg.openjdk.java.net/jdk/jdk/file/a44396903fd8/src/hotspot/share/services/virtualMemoryTracker.cpp#l351
>      >
>      > seems to suggest we do.
>      >
>      > However I believe we never enter this case since the region was
>     obtained
>      > using _reserved_regions->find(rgn); which only returns
>     overlapping regions,
>      > excluding neighbors.
>      >
>      > So IIUC currently we do not collapse adjacent regions. I think
>     this is also
>      > the correct behavior. Is above posted code snippet an overlook then?
>      >
>      > Or do we want to collapse regions? I think this makes not much sense
>      > though, since neighboring reservations may and do come from different
>      > callers with different memory flags.
>      >
>      > Thanks for clarifying,
>      >
>      > Thomas
>      >
>