Cache: Fix the LRU policy for classic memory hierarchy
The LRU policy always evicted the least recently touched way, even if it contained valid data and another way was invalid, as can happen if a block has been invalidated by coherance. This can result in caches never warming up even though they are replacing blocks. This modifies the LRU policy to move blocks to LRU position on invalidation.
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24
src/mem/cache/tags/cacheset.cc
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24
src/mem/cache/tags/cacheset.cc
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@ -66,3 +66,27 @@ CacheSet::moveToHead(CacheBlk *blk)
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} while (next != blk);
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}
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void
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CacheSet::moveToTail(CacheBlk *blk)
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{
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// nothing to do if blk is already tail
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if (blks[assoc-1] == blk)
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return;
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// write 'next' block into blks[i], moving from LRU to MRU
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// until we overwrite the block we moved to tail.
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// start by setting up to write 'blk' into tail
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int i = assoc - 1;
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CacheBlk *next = blk;
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do {
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assert(i >= 0);
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// swap blks[i] and next
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CacheBlk *tmp = blks[i];
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blks[i] = next;
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next = tmp;
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--i;
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} while (next != blk);
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}
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6
src/mem/cache/tags/cacheset.hh
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6
src/mem/cache/tags/cacheset.hh
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@ -66,6 +66,12 @@ class CacheSet
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*/
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void moveToHead(CacheBlk *blk);
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/**
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* Move the given block to the tail of the list.
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* @param blk The block to move
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*/
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void moveToTail(CacheBlk *blk);
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};
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#endif
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4
src/mem/cache/tags/lru.cc
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4
src/mem/cache/tags/lru.cc
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@ -213,6 +213,10 @@ LRU::invalidateBlk(BlkType *blk)
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blk->status = 0;
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blk->isTouched = false;
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blk->clearLoadLocks();
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// should be evicted before valid blocks
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unsigned set = blk->set;
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sets[set].moveToTail(blk);
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}
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}
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