On Tue, 2022-04-26 at 14:37 +0530, Aneesh Kumar K V wrote: > On 4/26/22 1:25 PM, ying.huang@xxxxxxxxx wrote: > > On Mon, 2022-04-25 at 16:45 +0530, Jagdish Gediya wrote: > > > On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@xxxxxxxxx wrote: > > > > On Sat, 2022-04-23 at 01:25 +0530, Jagdish Gediya wrote: > > > > > Some systems(e.g. PowerVM) can have both DRAM(fast memory) only > > > > > NUMA node which are N_MEMORY and slow memory(persistent memory) > > > > > only NUMA node which are also N_MEMORY. As the current demotion > > > > > target finding algorithm works based on N_MEMORY and best distance, > > > > > it will choose DRAM only NUMA node as demotion target instead of > > > > > persistent memory node on such systems. If DRAM only NUMA node is > > > > > filled with demoted pages then at some point new allocations can > > > > > start falling to persistent memory, so basically cold pages are in > > > > > fast memor (due to demotion) and new pages are in slow memory, this > > > > > is why persistent memory nodes should be utilized for demotion and > > > > > dram node should be avoided for demotion so that they can be used > > > > > for new allocations. > > > > > > > > > > Current implementation can work fine on the system where the memory > > > > > only numa nodes are possible only for persistent/slow memory but it > > > > > is not suitable for the like of systems mentioned above. > > > > > > > > Can you share the NUMA topology information of your machine? And the > > > > demotion order before and after your change? > > > > > > > > Whether it's good to use the PMEM nodes as the demotion targets of the > > > > DRAM-only node too? > > > > > > $ numactl -H > > > available: 2 nodes (0-1) > > > node 0 cpus: 0 1 2 3 4 5 6 7 > > > node 0 size: 14272 MB > > > node 0 free: 13392 MB > > > node 1 cpus: > > > node 1 size: 2028 MB > > > node 1 free: 1971 MB > > > node distances: > > > node 0 1 > > > 0: 10 40 > > > 1: 40 10 > > > > > > 1) without N_DEMOTION_TARGETS patch series, 1 is demotion target > > > for 0 even when 1 is DRAM node and there is no demotion targets for 1. > > > > > > $ cat /sys/bus/nd/devices/dax0.0/target_node > > > 2 > > > $ > > > # cd /sys/bus/dax/drivers/ > > > :/sys/bus/dax/drivers# ls > > > device_dax kmem > > > :/sys/bus/dax/drivers# cd device_dax/ > > > :/sys/bus/dax/drivers/device_dax# echo dax0.0 > unbind > > > :/sys/bus/dax/drivers/device_dax# echo dax0.0 > ../kmem/new_id > > > :/sys/bus/dax/drivers/device_dax# numactl -H > > > available: 3 nodes (0-2) > > > node 0 cpus: 0 1 2 3 4 5 6 7 > > > node 0 size: 14272 MB > > > node 0 free: 13380 MB > > > node 1 cpus: > > > node 1 size: 2028 MB > > > node 1 free: 1961 MB > > > node 2 cpus: > > > node 2 size: 0 MB > > > node 2 free: 0 MB > > > node distances: > > > node 0 1 2 > > > 0: 10 40 80 > > > 1: 40 10 80 > > > 2: 80 80 10 > > > > > > > This looks like a virtual machine, not a real machine. That's > > unfortunate. I am looking forward to a real issue, not a theoritical > > possible issue. > > > > This is the source of confusion i guess. A large class of ppc64 systems > are virtualized. The firmware include a hypervisor (PowerVM) and end > user creates guest (aka LPAR) on them. That is the way end user will use > this system. There is no baremetal access on this (There is an openpower > variant, but all new systems built by IBM these days do have PowerVM on > them). > > > So this is not a theoretical possibility. > Now I get it. Thanks for detailed explanation. Best Regards, Huang, Ying
