RFR(S) 8178351 - Simplify MetaspaceShared::is_in_shared_space and MetaspaceObj::is_shared

[Jiangli Zhou]

Hi Ioi,

Looks good!

In is_in_shared_metaspace(), can you please add an assert(p !=NULL, “can’t be NULL”)? When the shared spaces are not mapped, MetaspaceObj::_shared_metaspace_base and MetaspaceObj::_shared_metaspace_top are both NULL. In which case a NULL pointer could be mistakenly determined as in shared metaspace.

Thanks,
Jiangli

> On Jan 15, 2018, at 4:40 PM, Ioi Lam <ioi.lam at oracle.com> wrote:
> 
> Hi,
> 
> Please review the following simple fix for improving CDS start-up time:
> 
> https://bugs.openjdk.java.net/browse/JDK-8178351
> http://cr.openjdk.java.net/~iklam/jdk11/8178351-simplify-is-shared.v01/
> 
> Since all the CDS shared metaspace regions are consecutively allocated,
> and we can guarantee there are no gaps where the OS can allocate new
> memory into, we can just do comparisons between 2 pointers. See
> allocations.hpp and metaspaceShared.hpp where these methods can be
> inlined to improve start-up time when CDS is enabled.
> 
> For a simple HelloWorld test
> 
>     java -Xshare:dump
>     perf stat -r java -Xshare:on -cp . HelloWorld > /dev/null
> 
> We see about 1% speed up on my dual-core Haswell box. As usual, YMMV.
> 
>     BEFORE: 187,757,752 cycles, 0.048169533 sec
>     AFTER:  185,770,302 cycles, 0.047497524 sec
> 
> I also did some incidental code clean up.
> 
> Testing with hs_tier{1,2,3} now.
> 
> Thanks
> - Ioi