serviceability agent : problems when using gcc LTO (link time optimization)

[Aleksei Voitylov]

Volker,

not a full answer, but here is some static size stats:

Server     x86_64  AArch64
regular     23M       20M
lto            17M       14M

Minimal   x86_64  AArch64
regular     4.9M      3.9M
lto            4.7M      3.6M

-Aleksei

On 15/01/2020 16:40, Volker Simonis wrote:
> While we are speaking about all the drawbacks of LTO, it's still not
> clear what the benefits are? In the very first mail Matthias mentioned
> that there might be performance improvements but that performance is
> not the main driving factor behind this initiative. So is it the
> reduced code size (Matthias mentioned something around ~10%)?
>
> It would be nice to see some real numbers on various platform for
> both, the performance improvements for native parts like JIT/GC as
> well as for the size reduction.
>
> Aleksei Voitylov <aleksei.voitylov at bell-sw.com
> <mailto:aleksei.voitylov at bell-sw.com>> schrieb am Di., 14. Jan. 2020,
> 09:54:
>
>
>     On 14/01/2020 19:57, Baesken, Matthias wrote:
>     > Hello  Magnus and Aleksei,  thanks for the input .
>     >
>     > The times you  provided really look like they make a big
>     difference  at least for people  often  building   minimal-vm  .
>     > Guess I have to measure myself a bit  (maybe the difference is
>     not that big on our linux s390x / ppc64(le) ) .
>     >
>     >> If the change to enable lto by default is proposed, what would
>     be the
>     >> recommended strategy for development?
>     >>
>     > Probably  we should a)   do not enable it by default but just
>     make sure it can be enabled easily and works  for  the minimal-vm   
>     That would be welcome. I have high hopes to LTO the VM some time by
>     default, and the tendency observed is that the compiler time overhead
>     for GCC becomes smaller. At the same time there is no reason why
>     vendors
>     that invested in testing and can absorb the build time hit could
>     provide
>     binaries with LTO built VMs by passing an additional option flag.
>     >   or  b)  take it easy to disable it for local development.
>     >
>     > Best regards, Matthias
>     >
>     >
>     >
>     >> Magnus, Matthias,
>     >>
>     >> for me, lto is a little heavyweight for development. x86_64
>     build time
>     >> with gcc 7:
>     >>
>     >> Server 1m32.484s
>     >> Server+Minimal 1m42.166s
>     >> Server+Minimal (--with-jvm-features="link-time-opt") 5m29.422s
>     >>
>     >> If the change to enable lto by default is proposed, what would
>     be the
>     >> recommended strategy for development?
>     >>
>     >> For ARM32 Minimal, please keep in mind that it's not uncommon
>     to disable
>     >> LTO plugin in commodity ARM32 gcc compiler distributions, so
>     for some it
>     >> does not matter what settings we have in OpenJDK. I believe
>     there could
>     >> be other reasons for that on top of build time (bugs?).
>     >>
>