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

开始

把BgBufferSync 的代码内容简略化，得到：

bool                    

BgBufferSync(void)                    

{                    

    ……                

                    

    /* Used to compute how far we scan ahead */                

    long        strategy_delta;        

    int            bufs_to_lap;    

    ……                

    /* Variables for the scanning loop proper */                

    int            num_to_scan;    

    int            num_written;    

    int            reusable_buffers;    

    ……                

    if (saved_info_valid)                

    {                

        ……            

                    

        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;        

            ……        

        }            

        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);        

            ……        

        }            

        else            

        {            

            ……        

            bufs_to_lap = NBuffers;        

        }            

                    

    }                

    else                

    {                

        ……            

        bufs_to_lap = NBuffers;            

    }                

    ……                

                    

    /*                

     * Now write out dirty reusable buffers, working forward from the                

     * next_to_clean point, until we have lapped the strategy scan, or cleaned                

     * enough buffers to match our estimate of the next cycle's allocation                

     * requirements, or hit the bgwriter_lru_maxpages limit.                

     */                

                    

    ……                

    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--;            

        ……            

    }                

                    

    ……                

    /* Return true if OK to hibernate */                

    return (bufs_to_lap == 0 && recent_alloc == 0);                

}                    

一开始的时候，bufs_to_lap =4096, 就是 4096*8K=32MB ，相当于 Shared_buffers 的数量。

然后如果对代码进行跟踪，可以发现 bufs_to_lap 一开始就是 4096，然后逐次减1。

[作者：技术者高健@博客园  mail: [email protected] ]

结束