出现page allocation failure. order:1, mode:0x20解决办法 最近有机器遇到一问题：nginx: page allocation failure. order:1, mode:0x20 Pid: 18448, comm: nginx Not tainted 2.6.32-431.17.1.el6.x86_64 #1以下是在百度上找到的信息：dmesg里报出这个信息，然后机器负载开始上升，而实际上用free可以看到还有大量的内存被buffer/cached因此不应该出这个问题才对，我的系统是centos6，上centos的论坛查了一圈，有人回说是内核的bug一个临时解决方案是sysctl -w vm.zone_reclaim_mode=1关于zone_reclaim_mode的定义kernel的文档里描述如下Zone_reclaim_mode allows someone to set more or less aggressive approaches to reclaim memory when a zone runs out of memory. If it is set to zero then no zone reclaim occurs. Allocations will be satisfied from other zones / nodes in the system.This is value ORed together of1 = Zone reclaim on 2 = Zone reclaim writes dirty pages out 4 = Zone reclaim swaps pageszone_reclaim_mode is set during bootup to 1 if it is determined that pages from remote zones will cause a measurable performance reduction. The page allocator will then reclaim easily reusable pages (those page cache pages that are currently not used) before allocating off node pages.It may be beneficial to switch off zone reclaim if the system is used for a file server and all of memory should be used for caching files from disk. In that case the caching effect is more important than data locality.Allowing zone reclaim to write out pages stops processes that are writing large amounts of data from dirtying pages on other nodes. Zone reclaim will write out dirty pages if a zone fills up and so effectively throttle the process. This may decrease the performance of a single processsince it cannot use all of system memory to buffer the outgoing writes anymore but it preserve the memory on other nodes so that the performance of other processes running on other nodes will not be affected.Allowing regular swap effectively restricts allocations to the local node unless explicitly overridden by memory policies or cpuset configurations. 这个参数告诉内核当内存不够用时就直接回收buffer/cache