I wrote a script to find the "holes" in the class metadata structures caused by 8-byte alignment of pointer fields on 64-bit VMs. See attached run_pahole.sh script. It uses the Linux "pahole" to find the holes.

Results are in the attached file paholes.txt. Highlights are here. Fixing Method and MethodData should yield the most gain (about 1.5% saving of all class meta data for MedRec).

===== Klass ==== (save 8 bytes)

/* XXX 4 bytes hole, try to pack */

/* typedef jlong */ long int _last_biased_lock_bulk_revocation_time; /* 168 8 */
/* typedef markOop */ class markOopDesc * _prototype_header; /* 176 8 */
/* typedef jint */ int _biased_lock_revocation_count; /* 184 4 */

/* XXX 4 bytes hole, try to pack */

==== InstanceKlass === (save 8 bytes)

/* XXX 4 bytes hole, try to pack */

class JvmtiCachedClassFieldMap * _jvmti_cached_class_field_map; /* 416 8 */
bool _has_default_methods; /* 424 1 */

/* XXX 1 byte hole, try to pack */

volatile u2 _idnum_allocated_count; /* 426 2 */

/* XXX 4 bytes hole, try to pack */

=== Method === (save 8 bytes)
/* XXX 4 bytes hole, try to pack */

/* typedef jlong */ long int _prev_time; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
float _rate; /* 64 4 */

/* XXX 4 bytes hole, try to pack */
=== MethodData === (save 4 bytes)
/* typedef uint */ unsigned int _nof_overflow_traps; /* 32 4 */

/* XXX 4 bytes hole, try to pack */