The core idea behind the new parallel marking algorithm is simple. Instead of scanning individual objects, the garbage collector scans memory in much larger, contiguous blocks. The shared work queue tracks these coarse blocks instead of individual objects, and the individual objects waiting to be scanned in a block are tracked in that block itself. The core hypothesis is that while a block waits on the queue to be scanned, it will accumulate more objects to be scanned within that block, such that when a block does get dequeued, it’s likely that scanning will be able to scan more than one object in that block. This, in turn, improves locality of memory access, in addition to better amortizing per-scan costs.

source: L