Could we get rid of the UseCompressedClasses switch altogether, defaulting to const true?

[Thomas Stüfe]

Okay, we found a second reason why someone may choose
-XX:-UseCompressedClassPointers, at least in theory. Using +CCP will, of
course, increase the vsize of the process since we reserve the address
space for class space up front; without CCP, we would not use as much
address space.

Of course, vsize is pretty much free on modern Operating Systems, and
therefore this metric does not matter much. Modern 64-bit apps have
gigantic vsizes; java does too, the 1..3GB of class space are usually just
a drop in the sea here. Therefore, on modern Linux distros, vsize is
usually unlimited (ulimit -v).

And we talk not about 32-bit platforms, where class space does not exist,
so we don't intent to change anything there. This proposal only affects
64-bit platforms.

Disabling compressed class space in order to save vsize seems a bad
tradeoff to me: spending real physical memory (more heap) in order to
reduce address space.

Cheers, Thomas

On Thu, Feb 22, 2024 at 3:34 PM Thomas Stüfe <thomas.stuefe at gmail.com>
wrote:

> Greetings,
>
> Are there any realistic scenarios that would require
> UseCompressedClassPointers=false? Or could we make it constant at some
> point or remove the switch altogether?
>
> The only reason I can see for UseCompressedClassPointers=off would be if
> the maximum class space size of 3GB is not enough. But how realistic is
> this really? Some numbers:
>
> On average, a Klass is ~600-700 bytes. Therefore, 3GB of class space would
> be enough for ~5 million classes. Note that the typical size ratio of
> Klass-to-non-class-metadata is 1:6 .. 1:10, which means that 3 GB class
> space come with ~18..30 GB of non-class metaspace (see [1]). I think these
> are outlandish numbers. Do we need to support these scenarios? Some weird
> class generator scenarios maybe?
>
> Other than class space max size not being enough, are there any reasons
> for -UseCompressedClassPointers?
>
> Thank you,
>
> Thomas
>
> [1]
> https://github.com/tstuefe/metaspace-statistics/blob/master/Histogram.svg
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20240223/b40a8257/attachment.htm>