对PostgreSQL中bufmgr.c 中 bufs_to_lap的初步理解

开始,在代码中加入调试信息。无关部分设省略。

bool                                
BgBufferSync(void)                                
{                                
    ……                                                            
    int            bufs_to_lap;                
    ……                            
    if (saved_info_valid)                            
    {
        //added by gaojian
        fprintf(stderr,"saved_info_valid true\n");
                            
        int32        passes_delta = strategy_passes - prev_strategy_passes;                                        strategy_delta = strategy_buf_id - prev_strategy_buf_id;                        
        strategy_delta += (long) passes_delta *NBuffers;   
        ......                     
        if ((int32) (next_passes - strategy_passes) > 0)                        
        {                        
            fprintf(stderr,"next_pass > strategy_passes.\n");
            /* we're one pass ahead of the strategy point */                    
            bufs_to_lap = strategy_buf_id - next_to_clean;                    
            ……                    
        }                        
        else if (next_passes == strategy_passes &&                        
                 next_to_clean >= strategy_buf_id)                
        {
            fprintf(stderr,"next_passes == strategy_passes.\n");                         
            /* on same pass, but ahead or at least not behind */                    
            bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);                    
            ……                    
        }                        
        else                        
        {                        
            fprintf(stderr,"we are behind.\n"); 
            /*                    
             * We're behind, so skip forward to the strategy point and start                    
             * cleaning from there.                    
             */                    
            next_to_clean = strategy_buf_id;                    
            next_passes = strategy_passes;                    
            bufs_to_lap = NBuffers;                    
            ……                    
        }                 
    }                            
    else                            
    {  
        fprintf(stderr,"saved_info_valid false\n");                          
        ……                        
        bufs_to_lap = NBuffers;                        
    }                            
    ……                            
    bufs_ahead = NBuffers - bufs_to_lap;                            
    ……                            
    num_to_scan = bufs_to_lap; 
    ......                           
    /* Execute the LRU scan */                            
    while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)                            
    {                            
        //added by gaojian                        
        fprintf(stderr,"num_to_scan is: %d \n",num_to_scan);                        
                                
        int    buffer_state = SyncOneBuffer(next_to_clean, true);                    
        if (++next_to_clean >= NBuffers)                        
        {                        
            next_to_clean = 0;           
            elog(INFO,"------------------next_passes++.\n");                    
            next_passes++;                    
        }                        
        num_to_scan--;                        
        ……                        
    }                            
    ……                            
    new_strategy_delta = bufs_to_lap - num_to_scan;                            
    new_recent_alloc = reusable_buffers - reusable_buffers_est;                            
    if (new_strategy_delta > 0 && new_recent_alloc > 0)                            
    {                            
        scans_per_alloc = (float) new_strategy_delta / (float) new_recent_alloc;                        
        smoothed_density += (scans_per_alloc - smoothed_density) /                        
            smoothing_samples;                    
        ……                        
    }
    ......                            
}                                

运行的结果会如何呢？ 

[postgres@localhost bin]$ ./postgres -D /usr/local/pgsql/data
LOG:  database system was shut down at 2012-11-02 13:51:46 CST
saved_info_valid false.
LOG:  autovacuum launcher started
LOG:  database system is ready to accept connections
saved_info_valid true.
next_passes == strategy_passes.
saved_info_valid true.
next_passes == strategy_passes.
saved_info_valid true.
next_passes == strategy_passes.
saved_info_valid true.
next_passes == strategy_passes.
saved_info_valid true.
......

也就是说，一开始  saved_info_valid 是 false, 后来经过一次运行后，其值才发生转变，变成 true。

而之后， next_passes == startegy_passes  （其实，一开始都是零）

结束