RFR(xs): 8210068: Unsafe.allocateMemory() should not round up requested memory size

[Thomas Stüfe]

Hi all,

I closed that bug as wontfix. It is harmless enough to live with it.
Thanks for looking into this!

..Thomas

On Mon, Nov 5, 2018 at 8:13 AM Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
>
> Hi all,
>
> I am cleaning up my bug queue for 12.
>
> may I please have reviews for this small fix:
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8210068
> cr: http://cr.openjdk.java.net/~stuefe/webrevs/8210068-Unsafe.allocateMemory-shall-not-round-up-user-size/webrev.01/webrev/
>
> In Unsafe_AllocateMemory0(), we align the size requested by the caller
> up like this:
>
>   sz = align_up(sz, HeapWordSize);
>   void* x = os::malloc(sz, mtOther);
>
> This rounding is unnecessary, since the additional space is not needed
> by the caller. In addition, this rounding causes NMT to over-report
> the numbers of mtOther - which consists mainly of nio DirectByteBuffer
> related allocations - the more buffers and the smaller they are the
> more inaccurate those numbers become.
>
> --
>
> This bug has been originally reported by Adam Farley, see fragments of
> a scattered discussion on core-libs:
>
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-July/054412.html
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-August/054797.html
>
> --
>
> The patch ran through jdk-submit. I will put it through our
> SAP-internal tests to check whether removal of that alignment space
> could possibly trigger any hidden overwrite bugs.
>
> Thanks, Thomas