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。

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