On demand access check generation

ABSTRACT We des ribe an implementation of Ja kal, a Java based DSM where oheren y is enfor ed by ompiler inserted a ess he ks. To redu e the a ess he k overhead, the ompiler will rst try to redu e their number. Next the ompiler will insert lightweight he ks that rely on the hardware MMU. When a lightweight he k is triggered, it is (at runtime) hanged to a larger software only he k requiring no MMU support. This balan es the tradeo between high osts if not needed (many instru tions) in ase of a full software a ess he k and low ost if needed (no page fault) while a lightweight he k has low osts if not needed (few/ heap instru tions) and high osts if needed (page fault). When a full software he k has been determined to be redundant, it may be reverted ba k to a lightweight a ess he k. We nd that using these te hniques we an redu e a ess he k overhead by 12 per ent or more depending on appli ation. Another optimization done by the Ja kal ompiler is re ognizing tree like stru tures by reating a heap approximation. When a tree is dete ted, a ess he ks to the tree leaf nodes are removed, interior node a ess he ks are hanged to lightweight a ess he ks and whole trees are transfered over the network instead of single obje ts, one at a time. When (at runtime) false sharing is dete ted, the ode image is pat hed to hange a ess he ks to the old root to normal a ess he ks, making the tree level below the root the new tree roots.