remove INCLUDE_NMT?

[Thomas Stüfe]

Hi Alexei,

I built minimal with and without nmt (linux x64). Libjvm file size
increased from 5.26M -> 5.33M, an increase of 74k.

I tried to measure footprint (HelloWorld with fixed preallocated heap and
metaspace and AlwaysPreTouch) but could not see a consistent result.
Numbers were pretty stable for multiple runs and fluctuated 100M+-300k, but
with no discernable pattern. So whatever NMT costs at runtime is hidden by
noise.

In theory, having NMT in the libjvm means you pay 64/32 KB on
64/32bit platforms for the NMT preinit hash table. That is malloced at
startup and the only thing that cannot be avoided. The rest of the data is
or can easily be allocated on demand. Note that the reason the preinit hash
table was not showing up on RSS may be that even though it's malloced it
does usually not get used much, so it remains untouched.

Are these numbers worrisome to you? After reading all answers, I am leaning
toward removing INCLUDE_NMT, but am unemotional. It is mainly a cleanup
thing, and we could do that differently.

Cheers, Thomas

On Thu, Jan 20, 2022 at 10:17 PM Aleksei Voitylov <
aleksei.voitylov at bell-sw.com> wrote:

> Hi,
>
> we still build and use Minimal in production for constrained
> environments. What is the impact of unconditional inclusion of NMT in
> terms of static footprint?
>
> If it is negligible, I'm fine with it becoming unconditional. If not,
> maybe it's best to have some weak guarantees where we fix such configs
> occasionally.
>
> -Aleksei
>
> On 20/01/2022 09:58, 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 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?
> >
> > Thanks, Thomas
>