Cache替换策略

LRU, Least Recently Used, LRU算法根据各block(cache line)使用的情况， 总是选择那个最长时间未被使用的block进行替换。这种策略比较好的反映了程序局部性规律。

gem5中该替换策略的代码：

void
LRURP::invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
const
{
    // Reset last touch timestamp
    std::static_pointer_cast<LRUReplData>(
        replacement_data)->lastTouchTick = Tick(0);
}

void
LRURP::touch(const std::shared_ptr<ReplacementData>& replacement_data) const
{
    // Update last touch timestamp
    std::static_pointer_cast<LRUReplData>(
        replacement_data)->lastTouchTick = curTick();
}

void
LRURP::reset(const std::shared_ptr<ReplacementData>& replacement_data) const
{
    // Set last touch timestamp
    std::static_pointer_cast<LRUReplData>(
        replacement_data)->lastTouchTick = curTick();
}

ReplaceableEntry*
LRURP::getVictim(const ReplacementCandidates& candidates) const
{
    // There must be at least one replacement candidate
    assert(candidates.size() > 0);

    // Visit all candidates to find victim
    ReplaceableEntry* victim = candidates[0];
    for (const auto& candidate : candidates) {
        // Update victim entry if necessary
        if (std::static_pointer_cast<LRUReplData>(
                    candidate->replacementData)->lastTouchTick <
                std::static_pointer_cast<LRUReplData>(
                    victim->replacementData)->lastTouchTick) {
            victim = candidate;
        }
    }

    return victim;
}

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