RFR(xs): 8244777: ClassLoaderStats VM Op uses constant hash value

[Jaroslav Bachorík]

Hi Thomas,

The change looks good to me.
Would it be possible to have an accompanying test? Not a blocking
request, though.

Thanks!

-JB-


On Tue, May 12, 2020 at 3:35 PM Daniel D. Daugherty
<daniel.daugherty at oracle.com> wrote:
>
> Adding hotspot-jfr-dev at ... since this bug is filed against hotspot/jfr...
>
> Dan
>
>
> On 5/12/20 2:33 AM, Thomas Stüfe wrote:
> > Hi,
> >
> > may I get reviews for this small fix:
> >
> > bug: https://bugs.openjdk.java.net/browse/JDK-8244777
> > webrev:
> > http://cr.openjdk.java.net/~stuefe/webrevs/8244777--classloaderstats-vm-op-uses-constant-hash-value/webrev.00/webrev/
> >
> > This fixes runtime issues with the ClassLoaderStats VM Operation - with 50K
> > classloaders it currently takes 1.5s, with this patch it takes ~13 ms. This
> > issue was reported at [1].
> >
> > ClassLoaderStats iterates the CLDG and accumulates statistics per class
> > loader over all CLDs. It uses a temporary ResourceHashTable for that.
> >
> > The hash function is wrong though. It has two errors which independently
> > from each other cause the hash value to be constant, so the
> > ResourceHashTable degenerates to a linked list.
> >
> > static unsigned oop_hash(oop const& s1) {
> >     unsigned hash = (unsigned)((uintptr_t)&s1);
> >     return hash ^ (hash >> LogMinObjAlignment);
> > }
> >
> > - we use &s1, so we calculate the hash based on the address of the key
> > carrier not the key, which is always the same
> > - we use LogMinObjAlignment instead of LogMinObjAlignmentInBytes, which is
> > usually 0, so we xor out the hash at the end.
> >
> > After playing with the corrected version I got better results; then I
> > swapped this implementation with a common integer hash function we also use
> > in zgc, and got even better results. For a hash table with 256 buckets and
> > 50000 entries I now get a max chain len of 230, which is pretty close to
> > the optimal ~200.
> >
> > Thanks, Thomas
> >
> > [1]
> > https://mail.openjdk.java.net/pipermail/hotspot-jfr-dev/2020-May/001430.html
>