 Most prior work assumes that objects have the same size, but in practice object sizes often vary by several orders of magnitude  .
<meaning-changed>  Most prior work assumes that objects have the same size, but in practice object sizes often vary by several orders of magnitude  .
 Most prior work assumes that objects have the same size, but in practice object sizes often vary by several orders of magnitude , where computing the optimal miss ratio (OPT) is known to be NP-hard .