On Fri, Sep 3, 2010 at 2:06 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, 30 Aug 2010 00:43:48 +0900 > Minchan Kim <minchan.kim@xxxxxxxxx> wrote: > > > Ying Han reported that backing aging of anon pages in no swap system > > causes unnecessary TLB flush. > > > > When I sent a patch(69c8548175), I wanted this patch but Rik pointed out > > and allowed aging of anon pages to give a chance to promote from inactive > > to active LRU. > > > > It has a two problem. > > > > 1) non-swap system > > > > Never make sense to age anon pages. > > > > 2) swap configured but still doesn't swapon > > > > It doesn't make sense to age anon pages until swap-on time. > > But it's arguable. If we have aged anon pages by swapon, VM have moved > > anon pages from active to inactive. And in the time swapon by admin, > > the VM can't reclaim hot pages so we can protect hot pages swapout. > > > > But let's think about it. When does swap-on happen? It depends on admin. > > we can't expect it. Nonetheless, we have done aging of anon pages to > > protect hot pages swapout. It means we lost run time overhead when > > below high watermark but gain hot page swap-[in/out] overhead when VM > > decide swapout. Is it true? Let's think more detail. > > We don't promote anon pages in case of non-swap system. So even though > > VM does aging of anon pages, the pages would be in inactive LRU for a long > > time. It means many of pages in there would mark access bit again. So access > > bit hot/code separation would be pointless. > > > > This patch prevents unnecessary anon pages demotion in not-swapon and > > non-configured swap system. Of course, it could make side effect that > > hot anon pages could swap out when admin does swap on. > > But I think sooner or later it would be steady state. > > So it's not a big problem. > > We could lose someting but gain more thing(TLB flush and unnecessary > > function call to demote anon pages). > > > > I used total_swap_pages because we want to age anon pages > > even though swap full happens. > > We don't have any quantitative data on the effect of these excess tlb > flushes, which makes it difficult to decide which kernel versions > should receive this patch. > > Help? Andrew: We observed the degradation on 2.6.34 compared to 2.6.26 kernel. The workload we are running is doing 4k-random-write which runs about 3-4 minutes. We captured the TLB shootsdowns before/after: Before the change: TLB: 29435 22208 37146 25332 47952 43698 43545 40297 49043 44843 46127 50959 47592 46233 43698 44690 TLB shootdowns [HSUM = 662798 ] After the change: TLB: 2340 3113 1547 1472 2944 4194 2181 1212 2607 4373 1690 1446 2310 3784 1744 1134 TLB shootdowns [HSUM = 38091 ] Also worthy to mention, we are running in fake numa system where each fake node is 128M size. That makes differences on the check inactive_anon_is_low() since the active/inactive ratio falls to 1. --Ying -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>