GEM5: Different cache latency for read and write access 设置不同的读写latency

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#read #write #gem5

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本文介绍如何在gem5模拟器中为缓存设置不同的读写延迟。通过应用特定补丁，可在gem5中分别指定读取和写入缓存的延迟时间，从而更精细地控制模拟行为。

参考:2072reviews,1809reviews

由于gem5的cache默认读写延迟都是hit_latency,故无法设置不同的延迟，经过摸索，终于找到了read/write latency的修改方法，主要参考上面链接中的patch,将patch中的内容修改到自己的gem5源码中，然后再编译一次即可。

该patch的介绍如下：

This patch allows specifying different cache latency for read and write access. (In the code, the hit_latency parameter is actually the read_latency)

经过咨询该补丁的作者，修改后的hit_latency的含义就是read_latency，write_latency即新增加的参数，可以在caches.py文件中配置。

这样就可以设置cache不同的读写延迟了。

关于diff的代码如下：

diff -r 6a043adb1e8d src/mem/cache/BaseCache.py
--- a/src/mem/cache/BaseCache.py    Thu Jul 11 13:56:05 2013 -0500
+++ b/src/mem/cache/BaseCache.py    Thu Jan 09 10:25:33 2014 +0100
@@ -50,6 +50,7 @@
     assoc = Param.Int("associativity")
     block_size = Param.Int("block size in bytes")
     hit_latency = Param.Cycles("The hit latency for this cache")
+    write_latency = Param.Cycles("The write latency for this cache")
     response_latency = Param.Cycles(
             "Additional cache latency for the return path to core on a miss");
     max_miss_count = Param.Counter(0,
diff -r 6a043adb1e8d src/mem/cache/base.hh
--- a/src/mem/cache/base.hh Thu Jul 11 13:56:05 2013 -0500
+++ b/src/mem/cache/base.hh Thu Jan 09 10:25:33 2014 +0100
@@ -251,6 +251,11 @@
     const Cycles hitLatency;

     /**
+     * The latency of a write in this device.
+     */
+    const Cycles writeLatency;
+
+    /**
      * The latency of sending reponse to its upper level cache/core on a
      * linefill. In most contemporary processors, the return path on a cache
      * miss is much quicker that the hit latency. The responseLatency parameter
diff -r 6a043adb1e8d src/mem/cache/base.cc
--- a/src/mem/cache/base.cc Thu Jul 11 13:56:05 2013 -0500
+++ b/src/mem/cache/base.cc Thu Jan 09 10:25:33 2014 +0100
@@ -73,6 +73,7 @@
                   MSHRQueue_WriteBuffer),
       blkSize(p->block_size),
       hitLatency(p->hit_latency),
+      writeLatency(p->write_latency),
       responseLatency(p->response_latency),
       numTarget(p->tgts_per_mshr),
       forwardSnoops(p->forward_snoops),
diff -r 6a043adb1e8d src/mem/cache/cache_impl.hh
--- a/src/mem/cache/cache_impl.hh   Thu Jul 11 13:56:05 2013 -0500
+++ b/src/mem/cache/cache_impl.hh   Thu Jan 09 10:25:33 2014 +0100
@@ -284,7 +284,16 @@
         uncacheableFlush(pkt);
         blk = NULL;
         lat = hitLatency;
-        return false;
+   // Update latency decided by if it is read or write
+   if(pkt->isWrite()) {
+       lat = writeLatency;
+   }
+   return false;
+    }
+
+    // Update latency decided by if it is read or write
+    if(pkt->isWrite()) {
+   lat = writeLatency;
     }

     int id = pkt->req->hasContextId() ? pkt->req->contextId() : -1;
@@ -928,7 +937,7 @@
                 // from lower level caches/memory to an upper level cache or
                 // the core.
                 completion_time = clockEdge(responseLatency) +
-                    (transfer_offset ? pkt->busLastWordDelay :
+                    writeLatency * clockPeriod() + (transfer_offset ? pkt->busLastWordDelay :
                      pkt->busFirstWordDelay);

                 assert(!target->pkt->req->isUncacheable());
@@ -1255,7 +1264,7 @@
     }

     blk->whenReady = clockEdge() + responseLatency * clockPeriod() +
-        pkt->busLastWordDelay;
+        writeLatency * clockPeriod() + pkt->busLastWordDelay;

     return blk;
 }
diff -r 6a043adb1e8d src/mem/cache/tags/lru.cc
--- a/src/mem/cache/tags/lru.cc Thu Jul 11 13:56:05 2013 -0500
+++ b/src/mem/cache/tags/lru.cc Thu Jan 09 10:25:33 2014 +0100
@@ -125,20 +125,20 @@
     delete [] sets;
 }

+// Modification to take in account the right access latency (read or write)
 LRU::BlkType*
 LRU::accessBlock(Addr addr, Cycles &lat, int master_id)
 {
     Addr tag = extractTag(addr);
     unsigned set = extractSet(addr);
     BlkType *blk = sets[set].findBlk(tag);
-    lat = hitLatency;
     if (blk != NULL) {
         // move this block to head of the MRU list
         sets[set].moveToHead(blk);
         DPRINTF(CacheRepl, "set %x: moving blk %x to MRU\n",
                 set, regenerateBlkAddr(tag, set));
         if (blk->whenReady > curTick()
-            && cache->ticksToCycles(blk->whenReady - curTick()) > hitLatency) {
+            && cache->ticksToCycles(blk->whenReady - curTick()) > lat) {
             lat = cache->ticksToCycles(blk->whenReady - curTick());
         }
         blk->refCount += 1;