我对bufmgr.c 里的机理的粗浅理解

        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;
        }

    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
        {
             ......
        }
    }

    /* 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++;
    }

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