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/dataLOG:  database system was shut down at 2012-11-02 13:51:46 CSTsaved_info_valid false.LOG:  autovacuum launcher startedLOG:  database system is ready to accept connectionssaved_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