On 10/5/17 2:35 AM, Magnus Ihse Bursie wrote:
On 2017-10-05 11:07, Claes Redestad wrote:
On 2017-10-05 10:59, Magnus Ihse Bursie wrote:
How often is -Xbootclasspath/p used?
Why not use "-XX:-VerifySharedSpaces
-XX:SharedArchiveFile=local.jsa -Xshare:auto"? That way you will
have the start-up benefit if possible.
My worry here is that -Xshare:auto will not work correctly if
-Xbootclasspath/p is used. Maybe someone can guarantee that this
will work and convince me that it will never fail, but I don't think
this risk is worth the marginal gain.
Using -Xshare:auto should mean any case where a CDS archive can't be
used (for whatever reason) should be silently ignored. I'd be more
worried if -Xshare:on didn't fail in this case!
But we're actively disabling verification of the CDS archive! How is
then CDS supposed to know that it contains code for core classes that
has been superseded using -Xbootclasspath/p?
Hi Magnus,
Now I understand your hesitation -- sorry the docs aren't clear about
how the CDS archive is validated.
+ The classpath validation is always done, and cannot be turned off. So
if -Xbootclasspath/p is used, the CDS archive will not be loaded.
+ After the CDS archive is loaded, we have a second step that does a
checksum over its contents. This step can be disabled using
-XX:-VerifySharedSpaces
In fact, -Xshare:auto has been the default for the client VM for many
JDK releases, and we have not had any issues. (JDK-8188105 will make
-Xshare:auto the default for server VM as well.)
So, it's safe to use the combination of "-XX:-VerifySharedSpaces
-XX:SharedArchiveFile=local.jsa -Xshare:auto"
Thanks
- Ioi
This was supposed to be a quick and simple patch to get a small, but
useful improvement. It's not worth a lot of investigation or fixes,
imho.
Repeating my suggestion I put as a comment in the RFE: add
-Xshare:auto but leave out the code to dump an archive in the build
(for now),
so that those of us who prepare our boot JDK to have CDS archive
generated can get the benefit from it.
Well then, can you then guarantee that this will not break when
replacing stuff using -Xbootclasspath/p? Because I don't want to be
debugging things when a slightly different version of the class was
*not* used as it should since an old cached version in CDS was picked
up instead. :-(
/Magnus
/Claes
