Stream_Prefetcher_Gem5_Codes.md

Stream Prefetcher Gem5 Code Reading

1. 一些参数

// Creates per-core stream tables 
// for upto 64 processor cores
static const uint32_t MaxContexts = 64;
// Number of entries in a stream table
uint32_t tableSize;    
 // Use the master-id to train the streams   
const bool useMasterId;
// Determines the number of prefetch 
// reuquests to be issued at a time
uint32_t degree;
// Determines the prefetch distance
uint32_t distance;

2. 一些定义

• Stream Direction

// Direction of stream for each 
// stream entry in the stream table
enum StreamDirection{
// For example - A, A+1, A+2
	ASCENDING = 1,
/ For example - A, A-1, A-2
	DESCENDING = -1,
	INVALID = 0
};

• Stream Status

// Status of a stream entry in the stream table.
enum StreamStatus{
	INV       = 0,
// Stream training is not over yet. Once trained 
// will move to MONITOR status
	TRAINING  = 1,
// Monitor and Request: Stream entry ready 
// for issuing prefetch requests
	MONITOR   = 2 
};

• Stream Table Entry

class StreamTableEntry {
class StreamTableEntry {
  public:
	int  LRU_index;
	 // Address that initiated the stream training
	Addr allocAddr;
	// First address of a stream
	Addr startAddr;
	// Last address of a stream
	Addr endAddr;
	// Direction of trained stream (Ascending or Descending)
	StreamDirection trainedDirection;
	// Status of the stream entry
	StreamStatus    status;
	// Stores the last two stream directions of an entry
	StreamDirection trendDirection[2];
};

3. 核心函数

3.1. Monitor状态

• 先判断是否落在该stream的检测区间(startAddr, endAddr), 并且判断stream的方向，ascending还是descending
• 根据prefetch degree发送prefetch requests
• 修改stream检测区间 --> (startAddr, endAddr + degree)或者(startAddr + degree, endAddr + degree)

switch (table[i]->status) {
case MONITOR:
	if(table[i]->trainedDirection == ASCENDING) {
		// Ascending order
		if((table[i]->startAddr < blk_addr ) 
			&& ( table[i]->endAddr > blk_addr)) {
			// Hit to a stream, which is monitored. Issue prefetch 
			// requests based on the degree and the direction
			for (uint8_t d = 1; d <= degree; d++) {
				Addr pf_addr = table[i]->endAddr + blkSize * d;
				addresses.push_back(AddrPriority(pf_addr,0));
			}
			if((table[i]->endAddr + blkSize * degree) 
					- table[i]->startAddr <= distance) {
				table[i]->endAddr   
					= table[i]->endAddr + blkSize * degree;
			} else {
				table[i]->startAddr 
					= table[i]->startAddr + blkSize * degree;
				table[i]->endAddr   
					= table[i]->endAddr   + blkSize * degree;
			}
			break;
		}
	} else if(table[i]->trainedDirection == DESCENDING) {
		// 同上
	} else{
		assert(0);
	}
	break;

3.2. Training状态

• 判断blockAddr是否位于这个stream的检测范围
• 一个stream可以存两个trend, 所以先设置两个trend
• 如果两个trend匹配，则转换到Monitor状态；否则reset

case TRAINING:
if ((abs(table[i]->allocAddr - blk_addr) 
		<= (distance/2) * blkSize) ){
	// Check whether the address is in +/- of distance
	if(table[i]->trendDirection[0] == INVALID){
		table[i]->trendDirection[0] = 
			(blk_addr - table[i]->allocAddr > 0) ? ASCENDING : DESCENDING;
	} else {
		assert(table[i]->trendDirection[1] == INVALID);
		table[i]->trendDirection[1] = 
			(blk_addr - table[i]->allocAddr > 0) ? ASCENDING : DESCENDING;
		if(table[i]->trendDirection[0] == table[i]->trendDirection[1]) {
			table[i]->trainedDirection = table[i]->trendDirection[0];
			table[i]->startAddr = table[i]->allocAddr;
			if(table[i]->trainedDirection != INVALID){
				// Based on the trainedDirection (+1:Ascending, -1:Descending)
				// update the end address of a stream
				table[i]->endAddr = blk_addr 
					+ (table[i]->trainedDirection) * blkSize * degree;
			}
			// Entry is ready for issuing prefetch requests
			table[i]->status = MONITOR;
		} else {
			resetEntry(table[i]);
		}
	}
	break;
}
break;

3.3. 没有匹配的Stream

• 如果没有stream匹配的话，就要分配一个新的Stream
• 有invalid的stream就用invalid；没有的话根据LRU替换
• 然后设置allocAddr, 切换到Training状态

int entry_id;
if(HIT_index!=tableSize) {  //hit
	entry_id = HIT_index;
} else if (INVALID_index!=tableSize) {
	//Existence of invalid streams
	assert(table[INVALID_index]->status == INV);
	table[INVALID_index]->status = TRAINING;
	table[INVALID_index]->allocAddr = blk_addr;
	entry_id = INVALID_index;
} else {
	//Replace the LRU stream-entry
	assert(table[LRU_index]->status!=INV);
	resetEntry(table[LRU_index]);
	table[LRU_index]->status = TRAINING;
	table[LRU_index]->allocAddr = blk_addr;
	entry_id = LRU_index;
}

References

• gem5-with-DPC-2-prefetcher/stream.hh at master · hfsken/gem5-with-DPC-2-prefetcher · GitHub