>-----Original Message----- >From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On >Behalf Of Michal Hocko >Sent: Thursday, April 25, 2019 3:54 PM >To: Du, Fan <fan.du@xxxxxxxxx> >Cc: akpm@xxxxxxxxxxxxxxxxxxxx; Wu, Fengguang <fengguang.wu@xxxxxxxxx>; >Williams, Dan J <dan.j.williams@xxxxxxxxx>; Hansen, Dave ><dave.hansen@xxxxxxxxx>; xishi.qiuxishi@xxxxxxxxxxxxxxx; Huang, Ying ><ying.huang@xxxxxxxxx>; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx >Subject: Re: [RFC PATCH 0/5] New fallback workflow for heterogeneous >memory system > >On Thu 25-04-19 07:41:40, Du, Fan wrote: >> >> >> >-----Original Message----- >> >From: Michal Hocko [mailto:mhocko@xxxxxxxxxx] >> >Sent: Thursday, April 25, 2019 2:37 PM >> >To: Du, Fan <fan.du@xxxxxxxxx> >> >Cc: akpm@xxxxxxxxxxxxxxxxxxxx; Wu, Fengguang ><fengguang.wu@xxxxxxxxx>; >> >Williams, Dan J <dan.j.williams@xxxxxxxxx>; Hansen, Dave >> ><dave.hansen@xxxxxxxxx>; xishi.qiuxishi@xxxxxxxxxxxxxxx; Huang, Ying >> ><ying.huang@xxxxxxxxx>; linux-mm@xxxxxxxxx; >linux-kernel@xxxxxxxxxxxxxxx >> >Subject: Re: [RFC PATCH 0/5] New fallback workflow for heterogeneous >> >memory system >> > >> >On Thu 25-04-19 09:21:30, Fan Du wrote: >> >[...] >> >> However PMEM has different characteristics from DRAM, >> >> the more reasonable or desirable fallback style would be: >> >> DRAM node 0 -> DRAM node 1 -> PMEM node 2 -> PMEM node 3. >> >> When DRAM is exhausted, try PMEM then. >> > >> >Why and who does care? NUMA is fundamentally about memory nodes >with >> >different access characteristics so why is PMEM any special? >> >> Michal, thanks for your comments! >> >> The "different" lies in the local or remote access, usually the underlying >> memory is the same type, i.e. DRAM. >> >> By "special", PMEM is usually in gigantic capacity than DRAM per dimm, >> while with different read/write access latency than DRAM. > >You are describing a NUMA in general here. Yes access to different NUMA >nodes has a different read/write latency. But that doesn't make PMEM >really special from a regular DRAM. Not the numa distance b/w cpu and PMEM node make PMEM different than DRAM. The difference lies in the physical layer. The access latency characteristics comes from media level. >There are few other people trying to >work with PMEM as NUMA nodes and these kind of arguments are repeating >again and again. So far I haven't really heard much beyond hand waving. >Please go and read through those discussion so that we do not have to go >throug the same set of arguments again. > >I absolutely do see and understand people want to find a way to use >their shiny NVIDIMs but please step back and try to think in more >general terms than PMEM is special and we have to treat it that way. >We currently have ways to use it as DAX device and a NUMA node then >focus on how to improve our NUMA handling so that we can get maximum >out >of the HW rather than make a PMEM NUMA node a special snow flake. > >Thank you. > >-- >Michal Hocko >SUSE Labs
