我对bufmgr.c 里的机理的粗浅理解

开始

bufmgr.c 中，对strategy_passes 与 next_passes 有判断处理的逻辑，

        if ((int32) (next_passes - strategy_passes) > 0)
        {
            /* we're one pass ahead of the strategy point */
            bufs_to_lap = strategy_buf_id - next_to_clean;
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                 next_passes, next_to_clean,
                 strategy_passes, strategy_buf_id,
                 strategy_delta, bufs_to_lap);
#endif
        }
        else if (next_passes == strategy_passes &&
                 next_to_clean >= strategy_buf_id)
        {
            /* on same pass, but ahead or at least not behind */
            bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                 next_passes, next_to_clean,
                 strategy_passes, strategy_buf_id,
                 strategy_delta, bufs_to_lap);
#endif
        }
        else
        {
            /*
             * We're behind, so skip forward to the strategy point and start
             * cleaning from there.
             */
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter behind: bgw %u-%u strategy %u-%u delta=%ld",
                 next_passes, next_to_clean,
                 strategy_passes, strategy_buf_id,
                 strategy_delta);
#endif
            next_to_clean = strategy_buf_id;
            next_passes = strategy_passes;
            bufs_to_lap = NBuffers;
        }

那么，到底运行的时候，会发生什么呢？

我给代码加上调试语句，看看 strategy_passes 和 next_passes 的值都是多少。

    if (saved_info_valid)
    {
        fprintf(stderr,"stragey_passes is %u   ",stragegy_passes);
        fprintf(stderr,"...next_passes is %u   \n",next_passes);
 
        int32        passes_delta = strategy_passes - prev_strategy_passes;

        strategy_delta = strategy_buf_id - prev_strategy_buf_id;
        strategy_delta += (long) passes_delta *NBuffers;

        Assert(strategy_delta >= 0);

        if ((int32) (next_passes - strategy_passes) > 0)
        {
             ......
        }
        else if (next_passes == strategy_passes &&
                 next_to_clean >= strategy_buf_id)
        {
             ......
        }
        else
        {
             ......
        }
    }

还有

    /* Execute the LRU scan */
    while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
    {
        int            buffer_state = SyncOneBuffer(next_to_clean, true);

        if (++next_to_clean >= NBuffers)
        {
            next_to_clean = 0;
            fprintf(stderr,"----------next_passes++\n");
            next_passes++;
        }
        num_to_scan--;

        if (buffer_state & BUF_WRITTEN)
        {
            reusable_buffers++;
            if (++num_written >= bgwriter_lru_maxpages)
            {
                BgWriterStats.m_maxwritten_clean++;
                break;
            }
        }
        else if (buffer_state & BUF_REUSABLE)
            reusable_buffers++;
    }

运行的结果

strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
strategy_passes is:0  ....next_passes is:0  
------------------next_passes++.
strategy_passes is:0  ....next_passes is:1  
strategy_passes is:0  ....next_passes is:1  
strategy_passes is:0  ....next_passes is:1  
strategy_passes is:0  ....next_passes is:1  

刚运行的时候，哗哗出一堆 strategy_passes is:0  ....next_passes is:0

后来出来的行就慢一些，大概几秒一个。经过了 next_passes++ 的运算以后，next_passes 开始增长了。

通过监控可知，在 stragegy_passes = next_passes =0 的时间段里，进入 next_passes++ 之前，各个值如下：

next_passes	next_to_clean	strategy_passses	strategy_buf_id	strategy_delta	bufs_to_lap
0	4080	0	0	0	16
0	4086	0	0	0	10
0	4092	0	0	0	4

[作者：技术者高健@博客园  mail: luckyjackgao@gmail.com ]

结束