Hi Jorgen Hansen. Thank you for joining this topic and share your thoughts. I'm sorry for late reply due to some major tasks of our team this week. >> On 24 Mar 2023, at 10.50, Kyungsan Kim <ks0204.kim@xxxxxxxxxxx> wrote: >> >>> On 24.03.23 10:27, Kyungsan Kim wrote: >>>>> On 24.03.23 10:09, Kyungsan Kim wrote: >>>>>> Thank you David Hinderbrand for your interest on this topic. >>>>>> >>>>>>>> >>>>>>>>> Kyungsan Kim wrote: >>>>>>>>> [..] >>>>>>>>>>> In addition to CXL memory, we may have other kind of memory in the >>>>>>>>>>> system, for example, HBM (High Bandwidth Memory), memory in FPGA card, >>>>>>>>>>> memory in GPU card, etc. I guess that we need to consider them >>>>>>>>>>> together. Do we need to add one zone type for each kind of memory? >>>>>>>>>> >>>>>>>>>> We also don't think a new zone is needed for every single memory >>>>>>>>>> device. Our viewpoint is the sole ZONE_NORMAL becomes not enough to >>>>>>>>>> manage multiple volatile memory devices due to the increased device >>>>>>>>>> types. Including CXL DRAM, we think the ZONE_EXMEM can be used to >>>>>>>>>> represent extended volatile memories that have different HW >>>>>>>>>> characteristics. >>>>>>>>> >>>>>>>>> Some advice for the LSF/MM discussion, the rationale will need to be >>>>>>>>> more than "we think the ZONE_EXMEM can be used to represent extended >>>>>>>>> volatile memories that have different HW characteristics". It needs to >>>>>>>>> be along the lines of "yes, to date Linux has been able to describe DDR >>>>>>>>> with NUMA effects, PMEM with high write overhead, and HBM with improved >>>>>>>>> bandwidth not necessarily latency, all without adding a new ZONE, but a >>>>>>>>> new ZONE is absolutely required now to enable use case FOO, or address >>>>>>>>> unfixable NUMA problem BAR." Without FOO and BAR to discuss the code >>>>>>>>> maintainability concern of "fewer degress of freedom in the ZONE >>>>>>>>> dimension" starts to dominate. >>>>>>>> >>>>>>>> One problem we experienced was occured in the combination of hot-remove and kerelspace allocation usecases. >>>>>>>> ZONE_NORMAL allows kernel context allocation, but it does not allow hot-remove because kernel resides all the time. >>>>>>>> ZONE_MOVABLE allows hot-remove due to the page migration, but it only allows userspace allocation. >>>>>>>> Alternatively, we allocated a kernel context out of ZONE_MOVABLE by adding GFP_MOVABLE flag. >>>>>> >>>>>>> That sounds like a bad hack :) . >>>>>> I consent you. >>>>>> >>>>>>>> In case, oops and system hang has occasionally occured because ZONE_MOVABLE can be swapped. >>>>>>>> We resolved the issue using ZONE_EXMEM by allowing seletively choice of the two usecases. >>>>>> >>>>>>> I once raised the idea of a ZONE_PREFER_MOVABLE [1], maybe that's >>>>>>> similar to what you have in mind here. In general, adding new zones is >>>>>>> frowned upon. >>>>>> >>>>>> Actually, we have already studied your idea and thought it is similar with us in 2 aspects. >>>>>> 1. ZONE_PREFER_MOVABLE allows a kernelspace allocation using a new zone >>>>>> 2. ZONE_PREFER_MOVABLE helps less fragmentation by splitting zones, and ordering allocation requests from the zones. >>>>>> >>>>>> We think ZONE_EXMEM also helps less fragmentation. >>>>>> Because it is a separated zone and handles a page allocation as movable by default. >>>>> >>>>> So how is it different that it would justify a different (more confusing >>>>> IMHO) name? :) Of course, names don't matter that much, but I'd be >>>>> interested in which other aspect that zone would be "special". >>>> >>>> FYI for the first time I named it as ZONE_CXLMEM, but we thought it would be needed to cover other extended memory types as well. >>>> So I changed it as ZONE_EXMEM. >>>> We also would like to point out a "special" zone aspeact, which is different from ZONE_NORMAL for tranditional DDR DRAM. >>>> Of course, a symbol naming is important more or less to represent it very nicely, though. >>>> Do you prefer ZONE_SPECIAL? :) >>> >>> I called it ZONE_PREFER_MOVABLE. If you studied that approach there must >>> be a good reason to name it differently? >>> >> >> The intention of ZONE_EXMEM is a separated logical management dimension originated from the HW diffrences of extended memory devices. >> Althought the ZONE_EXMEM considers the movable and frementation aspect, it is not all what ZONE_EXMEM considers. >> So it is named as it. > >Given that CXL memory devices can potentially cover a wide range of technologies with quite different latency and bandwidth metrics, will one zone serve as the management vehicle that you seek? If a system contains both CXL attached DRAM and, let say, a byte-addressable CXL SSD - both used as (different) byte addressable tiers in a tiered memory hierarchy, allocating memory from the ZONE_EXMEM doesn’t really tell you much about what you get. So the client would still need an orthogonal method to characterize the desired performance characteristics. I agree that a heterogeneous system would be able to adopt multiple types of extended memory devices. We think ZONE_EXMEM can apply different management algorithms for each extended memory type. What we think is ZONE_NORMAL : ZONE_EXMEM = 1 : N, where N is the number of HW device type. ZONE_NORMAL is for conventional DDR DRAM on DIMM F/F, while ZONE_EXMEM is for extended memories, CXL DRAM, CXL SSD, etc on other F/Fs such as EDSFF. We think the movable attribute is a requirement for CXL DRAM device. However, there are other SW points we are concerning - implicit allocation and unintended migration - with CXL HW differences. So, I'm not sure if it is possible or good to cover the matters by combination of ZONE_MOVABLE and ZONE_PREFER_MOVABLE design. Let me point out again, we proposed the ZONE_EXMEM for the special logical management of extended memory devices. Specifically, for the performance metric, we think it would be handled not in the zone, but in a node unit. >This method could be combined with a fabric independent zone such as ZONE_PREFER_MOVABLE to address the kernel allocation issue. At the same time, this new zone could also be useful in other cases, such as virtio-mem. We agree with your thought. Along with adoption of CXL memory pool and fabric, virtualization SW layers would be added. Considering not only baremetal OS, but memory inflation/deflation between baremetal OS and a hypervisor, we think ZONE_EXMEM can be useful as the identifier for CXL memory. > >Thanks, >Jorgen
