Linux内核的缓存

落日映苍穹つ 2022-10-02 00:31 145阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

**缓存为什么会有冷热？**

究其原因，是因为对于内存的访问，可能是CPU发起的，也可以是DMA设备发起的。

如果是CPU发起的，在CPU的硬件缓存中，就会保存相应的页内容。如果这个页本来没有存在于硬件缓存中，那么它的到来，势必会将原本为其他的页缓存的内容挤出硬件缓存。

但是，如果对于内存的访问是由DMA设备发起的，那么该页不会被CPU访问，就不需要在CPU的硬件缓存中进行缓存，也不会对已经缓存在硬件缓存中的页内容造成伤害。

在[Linux][]操作系统中，每个内存区域(Zone)都分配了hot cache和cold cache，hot cache用来缓存那些很可能被CPU的硬件缓存收纳了的页。

hot/cold cache只处理单页分配的情况。

1: /*
       2:  * Really, prep_compound_page() should be called from __rmqueue_bulk().  But
       3:  * we cheat by calling it from here, in the order > 0 path.  Saves a branch
       4:  * or two.
       5:  */
       6: static inline
       7: struct page *buffered_rmqueue(struct zone *preferred_zone,
       8:             struct zone *zone, int order, gfp_t gfp_flags,
       9:             int migratetype)
      10: {
      11:     unsigned long flags;
      12:     struct page *page;
      13:     int cold = !!(gfp_flags & __GFP_COLD);
      14:  
      15: again:
      16:     if (likely(order == 0)) {
      17:         struct per_cpu_pages *pcp;
      18:         struct list_head *list;
      19:  
      20:         local_irq_save(flags);
      21:         pcp = &this_cpu_ptr(zone->pageset)->pcp;
      22:         list = &pcp->lists[migratetype];
      23:         if (list_empty(list)) {
      24:             pcp->count += rmqueue_bulk(zone, 0,
      25:                     pcp->batch, list,
      26:                     migratetype, cold);
      27:             if (unlikely(list_empty(list)))
      28:                 goto failed;
      29:         }
      30:  
      31:         if (cold)
      32:             page = list_entry(list->prev, struct page, lru);
      33:         else
      34:             page = list_entry(list->next, struct page, lru);
      35:  
      36:         list_del(&page->lru);
      37:         pcp->count--;
      38:     } else {
      39:         if (unlikely(gfp_flags & __GFP_NOFAIL)) {
      40:             /*
      41:              * __GFP_NOFAIL is not to be used in new code.
      42:              *
      43:              * All __GFP_NOFAIL callers should be fixed so that they
      44:              * properly detect and handle allocation failures.
      45:              *
      46:              * We most definitely don't want callers attempting to
      47:              * allocate greater than order-1 page units with
      48:              * __GFP_NOFAIL.
      49:              */
      50:             WARN_ON_ONCE(order > 1);
      51:         }
      52:         spin_lock_irqsave(&zone->lock, flags);
      53:         page = __rmqueue(zone, order, migratetype);
      54:         spin_unlock(&zone->lock);
      55:         if (!page)
      56:             goto failed;
      57:         __mod_zone_page_state(zone, NR_FREE_PAGES, -(1 << order));
      58:     }
      59:  
      60:     __count_zone_vm_events(PGALLOC, zone, 1 << order);
      61:     zone_statistics(preferred_zone, zone, gfp_flags);
      62:     local_irq_restore(flags);
      63:  
      64:     VM_BUG_ON(bad_range(zone, page));
      65:     if (prep_new_page(page, order, gfp_flags))
      66:         goto again;
      67:     return page;
      68:  
      69: failed:
      70:     local_irq_restore(flags);
      71:     return NULL;
      72: }

buffered\_rmqueue用于从冷热分配器中分配单页的缓存页。

如果gfp\_flags中指定的\_\_GFP\_COLD，则从冷缓存中分配一页，否则，从热缓存中分配。

转载于:https://my.oschina.net/u/3585265/blog/3014809

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220113/8ccca890913c48b88228e9ab005c54c2.png
[Linux]: https://www.linuxprobe.com/