remove INCLUDE_NMT?

[David Holmes]

On 20/01/2022 6:13 pm, Thomas Stüfe wrote:
> Hi David,
> 
> On Thu, Jan 20, 2022 at 8:22 AM David Holmes <david.holmes at oracle.com 
> <mailto:david.holmes at oracle.com>> wrote:
> 
>     Hi Thomas,
> 
>     On 20/01/2022 4:58 pm, Thomas Stüfe wrote:
>      > Hi,
>      >
>      > we have NMT, and NMT can be disabled at build time. It can also
>     be disabled
>      > at runtime. NMT disabling at build time causes INCLUDE_NMT to be
>     false.
>      >
>      > So we have four scenarios:
>      >
>      > 1 nmt disabled at build (minimal VM)
>      > 2 nmt enabled, off
>      > 3 nmt enabled, summary mode
>      > 4 nmt enabled, detail mode
>      >
>      > Removing (1) would simplify code and be one less configuration to
>     test. Do
>      > we really need the ability to exclude NMT from build? We have other
>      > facilities which are not strictly necessary either but which we
>     cannot
>      > disable, e.g. UL. So it feels a bit arbitrary to take that much
>     care for
>      > NMT. Was there a reason for that?
> 
>     It comes from the Compact Profiles work in JDK 8 and the MinimalVM. I
>     would not put UL in the same category as NMT as UL is far more
>     generally
>     useful even in a MinimalVM (though UL did not exist in 8).
> 
> 
> Okay, fair point. I did not know about Compact Profiles.
> 
>      > It only matters for minimal builds, since I don't know any real
>     VM out
>      > there which does not come with NMT.
>      >
>      > NMT comes with a few static data, but those are only a few KB and
>     we could
>      > instead allocate them on demand. NMT is not difficult to port
>     either, which
>      > would be another reason to keep it optional.
>      >
>      > What do you think?
> 
>     I think you always need NMT present for your changes in JDK-8277822 to
>     be valid! :)
> 
> 
> Well, not necessarily. You lose fences in compact debug builds, but 
> other than that, it still works.

?? But you removed the os.cpp logic for guarded memory, relying on NMT 
to do that, and NMT may not be present.

David
-----

> 
>     I don't know who, if anyone, uses a minimal VM these days. If the NMT
>     code doesn't add much to the disk or memory footprint then I think we
>     could decide to make it non-optional.
> 
> 
> Let's hear others. It is not that important. I was mainly eyeing the 
> stub interface to MemTracker for INCLUDE_NMT=false, because I routinely 
> forget about that one when changing NMT and then break the minimal build.
> 
> Thanks, Thomas
> 
>     Cheers,
>     David
>     -----
> 
>      > Thanks, Thomas
>