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. Regards, Bob -- 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>
