开始
bool BgBufferSync(void) { …… /* * Information saved between calls so we can determine the strategy * point's advance rate and avoid scanning already-cleaned buffers. */ static bool saved_info_valid = false; static int prev_strategy_buf_id; static uint32 prev_strategy_passes; static int next_to_clean; static uint32 next_passes; /* Moving averages of allocation rate and clean-buffer density */ static float smoothed_alloc = 0; static float smoothed_density = 10.0; …… if (saved_info_valid) { …… strategy_delta = strategy_buf_id - prev_strategy_buf_id; strategy_delta += (long) passes_delta *NBuffers; …… } else { …… }
fprintf(stderr,"before saving, prev_strategy_buf_id is %d \n", prev_strategy_buf_id); /* Update saved info for next time */ prev_strategy_buf_id = strategy_buf_id; prev_strategy_passes = strategy_passes; saved_info_valid = true; …… fprintf(stderr,"prev_strategy_buf_id is %d \n", prev_strategy_buf_id); /* Return true if OK to hibernate */ return (bufs_to_lap == 0 && recent_alloc == 0); }
执行的结果显示，即使我用psql 变更了某行的值，直到最后，prev_strategy_buf_id 的值也没有发生根本性的变化。
[postgres@localhost bin]$ ./postgres -D /usr/local/pgsql/data LOG: database system was shut down at 2012-11-02 15:30:38 CST saved_info_valid false. before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 LOG: autovacuum launcher started LOG: database system is ready to accept connections before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 …… before saving,prev_strategy_buf_id is: 0 prev_strategy_buf_id is 0 LOG: received fast shutdown request LOG: aborting any active transactions LOG: autovacuum launcher shutting down LOG: shutting down LOG: database system is shut down [postgres@localhost bin]$
