Linux内核的冷热缓存

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。