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List:       openjdk-hotspot-runtime-dev
Subject:    Re: RFR: 8236847: CDS archive with 4K alignment unusable on machines with 64k pages [v2]
From:       Andrew Haley <aph () openjdk ! java ! net>
Date:       2021-02-28 14:15:46
Message-ID: -kZmMeNHzoac57UR8vzDojYn3d876PcCwYJNJ3yk9M0=.f4498e12-378b-4b79-ad80-de63e609985a () github ! com
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On Sun, 28 Feb 2021 14:08:37 GMT, Andrew Haley <aph@openjdk.org> wrote:

> > Before this fix, the alignment is defaulting to that of the build host. We would \
> > like to provide a way to produce a JDK distribution, with a pre generated CDS \
> > archive, where the alignment has the highest known value of any target host for \
> > maximum compatibility. The currently known such values are 64K for Linux and 16K \
> > for Mac. If we aren't going to allow the user (builder of OpenJDK) the free \
> > choice of any alignment anyway, would it make sense to limit the choice between \
> > something more abstract like "host" and "compatible" instead of listing explicit \
> > numbers? 
> > Regardless of how the option is constructed, it will need some explanation in \
> > doc/building.md. 
> > Finally there is the question of if "host" or "compatible" should be the default. \
> > I see good arguments for both sides, as long as there is an option to switch \
> > between the too that isn't too cryptic to understand.
> 
> > Before this fix, the alignment is defaulting to that of the build host. We would \
> > like to provide a way to produce a JDK distribution, with a pre generated CDS \
> > archive, where the alignment has the highest known value of any target host for \
> > maximum compatibility. The currently known such values are 64K for Linux and 16K \
> > for Mac. If we aren't going to allow the user (builder of OpenJDK) the free \
> > choice of any alignment anyway, would it make sense to limit the choice between \
> > something more abstract like "host" and "compatible" instead of listing explicit \
> > numbers?
> 
> That's problematic because it assumes we know all of the possible alignments.  At \
> the present time we think that 64 is the largest we'll ever encounter, but IMO that \
> isn't a great way to think about things. It would be very nice indeed if we didn't \
> have to edit OpenJDK for the next page size. I guess 4k, 16k, and 64k are all we'll \
> ever see, but I wouldn't bet the farm on it. 
> > Finally there is the question of if "host" or "compatible" should be the default. \
> > I see good arguments for both sides, as long as there is an option to switch \
> > between the too that isn't too cryptic to understand.
> 
> I would have thought that "host" made the most sense, but I don't really mind.

> _Mailing list message from [Andrew Haley](mailto:aph@redhat.com) on \
> [hotspot-runtime-dev](mailto:hotspot-runtime-dev@openjdk.java.net):_ 
> Hmm. I'm not convinced by making the numeric argument here a free field.
> I'd allow precisely two options, "4k" and "64k", and if any crazy person
> needs to add "1M" in the future, let them do so.

I now realize this was wrong, because at least 16k exists as well.

-------------

PR: https://git.openjdk.java.net/jdk/pull/2651


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