On 02/27/2017 07:26 AM, Bob Liu wrote: > On 2017/2/24 12:53, Jerome Glisse wrote: >> On Fri, Feb 24, 2017 at 09:06:19AM +0800, Bob Liu wrote: >>> On 2017/2/21 21:39, Anshuman Khandual wrote: >>>> On 02/21/2017 04:41 PM, Michal Hocko wrote: >>>>> On Fri 17-02-17 17:11:57, Anshuman Khandual wrote: >>>>> [...] >>>>>> * User space using mbind() to get CDM memory is an additional benefit >>>>>> we get by making the CDM plug in as a node and be part of the buddy >>>>>> allocator. But the over all idea from the user space point of view >>>>>> is that the application can allocate any generic buffer and try to >>>>>> use the buffer either from the CPU side or from the device without >>>>>> knowing about where the buffer is really mapped physically. That >>>>>> gives a seamless and transparent view to the user space where CPU >>>>>> compute and possible device based compute can work together. This >>>>>> is not possible through a driver allocated buffer. >>>>> >>>>> But how are you going to define any policy around that. Who is allowed >>>> >>>> The user space VMA can define the policy with a mbind(MPOL_BIND) call >>>> with CDM/CDMs in the nodemask. >>>> >>>>> to allocate and how much of this "special memory". Is it possible that >>>> >>>> Any user space application with mbind(MPOL_BIND) call with CDM/CDMs in >>>> the nodemask can allocate from the CDM memory. "How much" gets controlled >>>> by how we fault from CPU and the default behavior of the buddy allocator. >>>> >>>>> we will eventually need some access control mechanism? If yes then mbind >>>> >>>> No access control mechanism is needed. If an application wants to use >>>> CDM memory by specifying in the mbind() it can. Nothing prevents it >>>> from using the CDM memory. >>>> >>>>> is really not suitable interface to (ab)use. Also what should happen if >>>>> the mbind mentions only CDM memory and that is depleted? >>>> >>>> IIUC *only CDM* cannot be requested from user space as there are no user >>>> visible interface which can translate to __GFP_THISNODE. MPOL_BIND with >>>> CDM in the nodemask will eventually pick a FALLBACK zonelist which will >>>> have zones of the system including CDM ones. If the resultant CDM zones >>>> run out of memory, we fail the allocation request as usual. >>>> >>>>> >>>>> Could you also explain why the transparent view is really better than >>>>> using a device specific mmap (aka CDM awareness)? >>>> >>>> Okay with a transparent view, we can achieve a control flow of application >>>> like the following. >>>> >>>> (1) Allocate a buffer: alloc_buffer(buf, size) >>>> (2) CPU compute on buffer: cpu_compute(buf, size) >>>> (3) Device compute on buffer: device_compute(buf, size) >>>> (4) CPU compute on buffer: cpu_compute(buf, size) >>>> (5) Release the buffer: release_buffer(buf, size) >>>> >>>> With assistance from a device specific driver, the actual page mapping of >>>> the buffer can change between system RAM and device memory depending on >>>> which side is accessing at a given point. This will be achieved through >>>> driver initiated migrations. >>>> >>> >>> Sorry, I'm a bit confused here. >>> What's the difference with the Heterogeneous memory management? >>> Which also "allows to use device memory transparently inside any process >>> without any modifications to process program code." >> >> HMM is first and foremost for platform (like Intel) where CPU can not >> access device memory in cache coherent way or at all. CDM is for more >> advance platform with a system bus that allow the CPU to access device >> memory in cache coherent way. >> >> Hence CDM was design to integrate more closely in existing concept like >> NUMA. From my point of view it is like another level in the memory >> hierarchy. Nowaday you have local node memory and other node memory. >> In not too distant future you will have fast CPU on die memory, local >> memory (you beloved DDR3/DDR4), slightly slower but gigantic persistant >> memory and also device memory (all those local to a node). >> >> On top of that you will still have the regular NUMA hierarchy between >> nodes. But each node will have its own local hierarchy of memory. >> >> CDM wants to integrate with existing memory hinting API and i believe >> this is needed to get some experience with how end user might want to >> use this to fine tune their application. >> >> Some bit of HMM are generic and will be reuse by CDM, for instance the >> DMA capable memory migration helpers. Wether they can also share HMM >> approach of using ZONE_DEVICE is yet to be proven but it comes with >> limitations (can't be on lru or have device lru) that might hinder a >> closer integration of CDM memory with many aspect of kernel mm. >> >> >> This is my own view and it likely differ in some way from the view of >> the people behind CDM :) >> > > Got it, thank you for the kindly explanation. > And also thank you, John. Thanks Jerome and John for helping out with the detailed explanation. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
