.net framework bar code The Cache Hierarchy in Software Integration PDF417 in Software The Cache Hierarchy

The Cache Hierarchy use none none integrating topaint none in nonegeneration ean code c# In the presence of a vi none for none ctim cache, a memory reference proceeds as follows (we assume that physical addresses are used throughout and that the L1 cache is direct-mapped; it is easy to generalize to set-associative caches). First, the L1 cache is accessed. If there is a hit, we are done; otherwise, the line that would have been replaced in a regular direct-mapped cache becomes the victim.

Then the victim cache is probed. If there is a hit in the victim cache, the victim and the line hit in the victim cache are swapped. Thus, the most recently used reference in the set is in the direct-mapped cache.

If there is a miss in the victim cache, the line that is evicted from the regular cache, that is, the victim, is stored in the victim cache (if the latter is full, one of its lines is evicted to the next level in the memory hierarchy). When the miss is resolved in a higher level of the memory hierarchy, the missing line is sent to the regular cache..

Code11 VICTIM CACHE. An L1 cache access is decomposed into its index and tag ref. A victim cache none none (VC) access is an associative search of all tags in VC (array VC.tag) L1 Access.

if tag(index) = tag ref then begin hit; exit end/ hit in L1 /. VC Access victim line(index) / index is concatenated with the tag of the victim if (one of VC.tag[ ] = concat(tag ref,index), say i then begin swap (victim, VC[i]); / hit in VC / modify LRU(VC); exit end. Miss Select LRU line in VC, say j; Writeback(VC[j]); VC[j] victim;. Because the victim cach none for none e is fully associative, the tag of a victim cache entry consists of the whole physical address except for the d least signi cant bits, which are used for the displacement within a line. On a swap, the tag of the victim line in the cache will be concatenated with its index, and the index will be stripped from the line in the victim cache when replacing a line in the cache. In general, accessing (swapping) the victim cache takes one extra cycle, but it is worth it if misses can be substantially reduced.

Experiments show that small victim caches (say four to eight entries) are often quite effective in removing con ict misses for small (say less than 32 K) direct-mapped caches. The rst victim cache was implemented for the HP 7100. In its successor, the HP 7200, the victim cache has 64 entries and can be accessed as fast as a large off-chip L1 data cache of 1 MB (this is 1995 vintage with both the processor and the cache clocked at 120 MHz).

Larger victim caches have been proposed to assist L2 caches. However, a related concept, exclusive caching, is preferred. We shall return to the topic in Section 6.

3.. 6.1 Improving Access to L1 Caches 6.1.3 Code and Data Reo none none rdering In an out-of-order processor, data cache read misses might be tolerated if there are enough instructions not dependent on the data being read that can still proceed.

On an instruction cache miss, though, the front-end has to stall until the instructions can be retrieved, hopefully from the next level in the memory hierarchy. Thus implementing techniques to reduce as much as possible the number of I-cache misses would be very useful. In this section, we present one such technique, a softwarebased approach called code reordering, whose primary goal from the I-cache viewpoint is to reduce the number of potential con ict misses.

The basic idea in code reordering for improving cache performance, and also for reducing branch misprediction penalties, is to reorder procedures and basic blocks within procedures rather than letting the original compiler order be the default. The reordering is based on statistics gathered through pro ling. Therefore, the method can only be used for production programs; but this is not much of a deterrent, for production programs are those for which the best performance is needed.

Procedure reordering attempts to place code that is commonly used together close in space, thus reducing the number of con ict misses. For example, in the very simple case where procedure P is the only one to call procedure Q, it would be bene cial to have P and Q occupy consecutive portions of the I-cache. The input to the procedure reordering algorithm is a call graph, an undirected graph where the nodes represent the procedures and the weighted edges are the frequencies of calls between the procedures.

The most common algorithm is closest is best, using a greedy approach to do the placement. At each step of the algorithm, the two nodes connected by the edge of highest weight are merged. A merged node consists of an ordered list of all the procedures that compose it.

Remaining edges that leave the merged node are coalesced. Procedures, or sets of procedures, say A and B, that are selected are merged, taking into account which of the four possible orderings AB, Areverse B, ABreverse , Areverse Breverse yields the heaviest connection between the procedures at the boundaries of the two sets. Basic block reordering is nothing more than trying to maximize the number of fall-through branches.

From the cache viewpoint, its advantage is that increasing the length of uninterrupted code sequences allows a more ef cient prefetching of lines from the cache into the instruction buffer as well as prefetching between the cache and the next level in the memory hierarchy (see Section 6.2.1).

Additionally, because only taken branches are stored in the BTB, the latter can be better utilized. Finally, as we saw in 4, the correct prediction of taken branches requires an extra cycle in most deeply pipelined microarchitectures, a penalty that is not present for successfully predicted not-taken branches. Basic block reordering within a procedure proceeds in a manner similar to that of procedure reordering, except that now the call graph is a directed graph, that is, the positions of blocks within a merged node are dictated by the original control ow.

Further re nements, using techniques similar to page coloring but applied to.
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