mem: Make tXAW enforcement less conservative and per rank

This patch changes the tXAW constraint so that it is enforced per rank
rather than globally for all ranks in the channel. It also avoids
using the bank freeAt to enforce the activation limit, as doing so
also precludes performing any column or row command to the
DRAM. Instead the patch introduces a new variable actAllowedAt for the
banks and use this to track when a potential activation can occur.

1 files changed, 28 insertions, 16 deletions

diff --git a/src/mem/simple_dram.cc b/src/mem/simple_dram.cc
index 9669c7a03..bd700c09d 100644
--- a/src/mem/simple_dram.cc
+++ b/src/mem/simple_dram.cc
@@ -55,7 +55,7 @@ SimpleDRAM::SimpleDRAM(const SimpleDRAMParams* p) :
     AbstractMemory(p),
     port(name() + ".port", *this),
     retryRdReq(false), retryWrReq(false),
-    rowHitFlag(false), stopReads(false), actTicks(p->activation_limit, 0),
+    rowHitFlag(false), stopReads(false),
     writeEvent(this), respondEvent(this),
     refreshEvent(this), nextReqEvent(this), drainManager(NULL),
     deviceBusWidth(p->device_bus_width), burstLength(p->burst_length),
@@ -82,8 +82,10 @@ SimpleDRAM::SimpleDRAM(const SimpleDRAMParams* p) :
     // create the bank states based on the dimensions of the ranks and
     // banks
     banks.resize(ranksPerChannel);
+    actTicks.resize(ranksPerChannel);
     for (size_t c = 0; c < ranksPerChannel; ++c) {
         banks[c].resize(banksPerRank);
+        actTicks[c].resize(activationLimit, 0);
     }
 
     // round the write threshold percent to a whole number of entries
@@ -913,6 +915,7 @@ SimpleDRAM::estimateLatency(DRAMPacket* dram_pkt, Tick inTime)
     Tick accLat = 0;
     Tick bankLat = 0;
     rowHitFlag = false;
+    Tick potentialActTick;
 
     const Bank& bank = dram_pkt->bankRef;
     if (pageMgmt == Enums::open) { // open-page policy
@@ -946,6 +949,11 @@ SimpleDRAM::estimateLatency(DRAMPacket* dram_pkt, Tick inTime)
             if (freeTime > inTime)
                accLat += freeTime - inTime;
 
+            //The bank is free, and you may be able to activate
+            potentialActTick = inTime + accLat + tRP;
+            if (potentialActTick < bank.actAllowedAt)
+                accLat += bank.actAllowedAt - potentialActTick;
+
             accLat += tRP + tRCD + tCL;
             bankLat += tRP + tRCD + tCL;
         }
@@ -955,6 +963,11 @@ SimpleDRAM::estimateLatency(DRAMPacket* dram_pkt, Tick inTime)
         if (bank.freeAt > inTime)
             accLat += bank.freeAt - inTime;
 
+        //The bank is free, and you may be able to activate
+        potentialActTick = inTime + accLat;
+        if (potentialActTick < bank.actAllowedAt)
+            accLat += bank.actAllowedAt - potentialActTick;
+
         // page already closed, simply open the row, and
         // add cas latency
         accLat += tRCD + tCL;
@@ -975,42 +988,41 @@ SimpleDRAM::processNextReqEvent()
 }
 
 void
-SimpleDRAM::recordActivate(Tick act_tick)
+SimpleDRAM::recordActivate(Tick act_tick, uint8_t rank)
 {
-    assert(actTicks.size() == activationLimit);
+    assert(0 <= rank && rank < ranksPerChannel);
+    assert(actTicks[rank].size() == activationLimit);
 
     DPRINTF(DRAM, "Activate at tick %d\n", act_tick);
 
     // if the activation limit is disabled then we are done
-    if (actTicks.empty())
+    if (actTicks[rank].empty())
         return;
 
     // sanity check
-    if (actTicks.back() && (act_tick - actTicks.back()) < tXAW) {
+    if (actTicks[rank].back() && (act_tick - actTicks[rank].back()) < tXAW) {
         // @todo For now, stick with a warning
         warn("Got %d activates in window %d (%d - %d) which is smaller "
-             "than %d\n", activationLimit, act_tick - actTicks.back(),
-             act_tick, actTicks.back(), tXAW);
+             "than %d\n", activationLimit, act_tick - actTicks[rank].back(),
+             act_tick, actTicks[rank].back(), tXAW);
     }
 
     // shift the times used for the book keeping, the last element
     // (highest index) is the oldest one and hence the lowest value
-    actTicks.pop_back();
+    actTicks[rank].pop_back();
 
     // record an new activation (in the future)
-    actTicks.push_front(act_tick);
+    actTicks[rank].push_front(act_tick);
 
     // cannot activate more than X times in time window tXAW, push the
     // next one (the X + 1'st activate) to be tXAW away from the
     // oldest in our window of X
-    if (actTicks.back() && (act_tick - actTicks.back()) < tXAW) {
+    if (actTicks[rank].back() && (act_tick - actTicks[rank].back()) < tXAW) {
         DPRINTF(DRAM, "Enforcing tXAW with X = %d, next activate no earlier "
-                "than %d\n", activationLimit, actTicks.back() + tXAW);
-        for(int i = 0; i < ranksPerChannel; i++)
+                "than %d\n", activationLimit, actTicks[rank].back() + tXAW);
             for(int j = 0; j < banksPerRank; j++)
                 // next activate must not happen before end of window
-                banks[i][j].freeAt = std::max(banks[i][j].freeAt,
-                                              actTicks.back() + tXAW);
+                banks[rank][j].actAllowedAt = actTicks[rank].back() + tXAW;
     }
 }
 
@@ -1049,7 +1061,7 @@ SimpleDRAM::doDRAMAccess(DRAMPacket* dram_pkt)
             // any waiting for banks account for in freeAt
             actTick = bank.freeAt - tCL - tRCD;
             bank.tRASDoneAt = actTick + tRAS;
-            recordActivate(actTick);
+            recordActivate(actTick, dram_pkt->rank);
 
             // sample the number of bytes accessed and reset it as
             // we are now closing this row
@@ -1058,7 +1070,7 @@ SimpleDRAM::doDRAMAccess(DRAMPacket* dram_pkt)
         }
     } else if (pageMgmt == Enums::close) {
         actTick = curTick() + addDelay + accessLat - tRCD - tCL;
-        recordActivate(actTick);
+        recordActivate(actTick, dram_pkt->rank);
 
         // If the DRAM has a very quick tRAS, bank can be made free
         // after consecutive tCL,tRCD,tRP times. In general, however,