mem: Make DRAM write queue draining more aggressive

This patch changes the triggering condition for the write draining
such that we grab the opportunity to issue writes if there are no
reads waiting (as opposed to waiting for the writes to reach the high
threshold). As a result, we potentially drain some of the writes in read
idle periods (if any).

A low threshold is added to be able to control how many write bursts
are kept in the memory controller queue (acting as on-chip storage).

The high and low thresholds are updated to sensible values for a 32/64
size write buffer. Note that the thresholds should be adjusted along
with the queue sizes.

This patch also adds some basic initialisation sanity checks and moves
part of the initialisation to the constructor.

1 files changed, 5 insertions, 8 deletions

diff --git a/src/mem/simple_dram.hh b/src/mem/simple_dram.hh
index 0731b14bb..8ecce94b7 100644
--- a/src/mem/simple_dram.hh
+++ b/src/mem/simple_dram.hh
@@ -471,17 +471,17 @@ class SimpleDRAM : public AbstractMemory
     const uint32_t devicesPerRank;
     const uint32_t burstSize;
     const uint32_t rowBufferSize;
+    const uint32_t columnsPerRowBuffer;
     const uint32_t ranksPerChannel;
     const uint32_t banksPerRank;
     const uint32_t channels;
     uint32_t rowsPerBank;
-    uint32_t columnsPerRowBuffer;
     const uint32_t readBufferSize;
     const uint32_t writeBufferSize;
-    const double writeHighThresholdPerc;
-    uint32_t writeHighThreshold;
-    const double writeLowThresholdPerc;
-    uint32_t writeLowThreshold;
+    const uint32_t writeHighThreshold;
+    const uint32_t writeLowThreshold;
+    const uint32_t minWritesPerSwitch;
+    uint32_t writesThisTime;
 
     /**
      * Basic memory timing parameters initialized based on parameter
@@ -530,9 +530,6 @@ class SimpleDRAM : public AbstractMemory
     Tick prevArrival;
     int numReqs;
 
-    // Tracks number of writes done to meet the write threshold
-    uint32_t numWritesThisTime;
-
     // The absolute soonest you have to start thinking about the
     // next request is the longest access time that can occur before
     // busBusyUntil. Assuming you need to precharge,