On Wed, 2022-05-11 at 23:22 -0700, Wei Xu wrote: > > Memory Allocation for Demotion > ============================== > > To allocate a new page as the demotion target for a page, the kernel > calls the allocation function (__alloc_pages_nodemask) with the > source page node as the preferred node and the union of all lower > tier nodes as the allowed nodemask. The actual target node selection > then follows the allocation fallback order that the kernel has > already defined. > > The pseudo code looks like: > > targets = NODE_MASK_NONE; > src_nid = page_to_nid(page); > src_tier = node_tier_map[src_nid]; > for (i = src_tier + 1; i < MAX_MEMORY_TIERS; i++) > nodes_or(targets, targets, memory_tiers[i]); > new_page = __alloc_pages_nodemask(gfp, order, src_nid, targets); > > The memopolicy of cpuset, vma and owner task of the source page can > be set to refine the demotion target nodemask, e.g. to prevent > demotion or select a particular allowed node as the demotion target. Consider a system with 3 tiers, if we want to demote some pages from tier 0, the desired behavior is, - Allocate pages from tier 1 - If there's no enough free pages in tier 1, wakeup kswapd of tier 1 so demote some pages from tier 1 to tier 2 - If there's still no enough free pages in tier 1, allocate pages from tier 2. In this way, tier 0 will have the hottest pages, while tier 1 will have the coldest pages. With your proposed method, the demoting from tier 0 behavior is, - Allocate pages from tier 1 - If there's no enough free pages in tier 1, allocate pages in tier 2 The kswapd of tier 1 will not be waken up until there's no enough free pages in tier 2. In quite long time, there's no much hot/cold differentiation between tier 1 and tier 2. This isn't hard to be fixed, just call __alloc_pages_nodemask() for each tier one by one considering page allocation fallback order. Best Regards, Huang, Ying
