Berkeley DB Memory Pool LRU algorithm

Priority based LRU

src/mp/mp_fget.c, __memp_fget()in, when initializing a page buffer, set its priority:

    bhp->priority = MPOOL_LRU_REDZONE;    

In src/mp/mp_fget.c, __memp_fput() , put a page buffer and make it reference minus one. bhp->ref-- if reference is reduced to 0, its priority is adjusted. The following bhp->priority = c_mp->lru_priority; and ++c_mp->lru_priority guaranteed LRU; And according to the incoming Priorit The y parameter is adjusted accordingly.
Prioriry Max is Uint32_max, so it may be necessary to wraparound. __MEMP_RESET_LRU () lowers the priority of all page buffer.

/* Update priority values. */if (priority = = Db_priority_very_low | | Mfp->priority = = mpool_pri_very_low) bhp->priority =0;else {/* * We don ' t lock the LRU priority or the pages field, if * we get garbage (which won ' t happen on a 32-bit machine), it * Only means a buffer had the wrong priority. */bhp->priority = c_mp->lru_priority;Switch (priority) {DefaultCase Db_priority_unchanged:pfactor = mfp->priority;BreakCase db_priority_very_low:pfactor = Mpool_pri_very_low;BreakCase db_priority_low:pfactor = Mpool_pri_low;BreakCase db_priority_default:pfactor = Mpool_pri_default;Breakcase db_priority_high:pfactor = Mpool_pri_high; break; case db_priority_very_high:pfactor = Mpool_pri_very_high; break;} adjust = 0; if (pfactor! = 0) adjust = (int) c_mp->pages/pfactor; if (F_isset (BHP, Bh_dirty)) Adjust + = (int) c_mp->pages/mpool_pri_dirty; if (Adjust > 0) {if (MPOOL_LRU_ Redzone-bhp->priority >= (u_int32_t) adjust) bhp->priority + = adjust; } else if (Adjust < 0) if (Bhp->priority > (u_int32_t)-adjust) bhp->priority + = adjust;}     

...

/* * On every buffer put we update the cache lru priority and check * for wraparound. The increment doesn‘t need to be atomic: occasional * lost increments are okay; __memp_reset_lru handles race conditions. */if (++c_mp->lru_priority >= MPOOL_LRU_REDZONE &&    (t_ret = __memp_reset_lru(env, infop)) != 0 && ret == 0)    ret = t_ret;return (ret);

src/mp/mp_alloc.c, __memp_alloc(), the existing buffer memory is reused when the memory of the new buffer cannot be allocated. The algorithm is very complex, write one alone.

Berkeley DB Memory Pool LRU algorithm