On Thu, Apr 8, 2021 at 7:58 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Yang Shi <shy828301@xxxxxxxxx> writes: > > > On Thu, Apr 8, 2021 at 10:19 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > >> > >> Hi Tim, > >> > >> On Mon, Apr 5, 2021 at 11:08 AM Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote: > >> > > >> > Traditionally, all memory is DRAM. Some DRAM might be closer/faster than > >> > others NUMA wise, but a byte of media has about the same cost whether it > >> > is close or far. But, with new memory tiers such as Persistent Memory > >> > (PMEM). there is a choice between fast/expensive DRAM and slow/cheap > >> > PMEM. > >> > > >> > The fast/expensive memory lives in the top tier of the memory hierachy. > >> > > >> > Previously, the patchset > >> > [PATCH 00/10] [v7] Migrate Pages in lieu of discard > >> > https://lore.kernel.org/linux-mm/20210401183216.443C4443@xxxxxxxxxxxxxxxxxx/ > >> > provides a mechanism to demote cold pages from DRAM node into PMEM. > >> > > >> > And the patchset > >> > [PATCH 0/6] [RFC v6] NUMA balancing: optimize memory placement for memory tiering system > >> > https://lore.kernel.org/linux-mm/20210311081821.138467-1-ying.huang@xxxxxxxxx/ > >> > provides a mechanism to promote hot pages in PMEM to the DRAM node > >> > leveraging autonuma. > >> > > >> > The two patchsets together keep the hot pages in DRAM and colder pages > >> > in PMEM. > >> > >> Thanks for working on this as this is becoming more and more important > >> particularly in the data centers where memory is a big portion of the > >> cost. > >> > >> I see you have responded to Michal and I will add my more specific > >> response there. Here I wanted to give my high level concern regarding > >> using v1's soft limit like semantics for top tier memory. > >> > >> This patch series aims to distribute/partition top tier memory between > >> jobs of different priorities. We want high priority jobs to have > >> preferential access to the top tier memory and we don't want low > >> priority jobs to hog the top tier memory. > >> > >> Using v1's soft limit like behavior can potentially cause high > >> priority jobs to stall to make enough space on top tier memory on > >> their allocation path and I think this patchset is aiming to reduce > >> that impact by making kswapd do that work. However I think the more > >> concerning issue is the low priority job hogging the top tier memory. > >> > >> The possible ways the low priority job can hog the top tier memory are > >> by allocating non-movable memory or by mlocking the memory. (Oh there > >> is also pinning the memory but I don't know if there is a user api to > >> pin memory?) For the mlocked memory, you need to either modify the > >> reclaim code or use a different mechanism for demoting cold memory. > > > > Do you mean long term pin? RDMA should be able to simply pin the > > memory for weeks. A lot of transient pins come from Direct I/O. They > > should be less concerned. > > > > The low priority jobs should be able to be restricted by cpuset, for > > example, just keep them on second tier memory nodes. Then all the > > above problems are gone. > > To optimize the page placement of a process between DRAM and PMEM, we > want to place the hot pages in DRAM and the cold pages in PMEM. But the > memory accessing pattern changes overtime, so we need to migrate pages > between DRAM and PMEM to adapt to the changing. > > To avoid the hot pages be pinned in PMEM always, one way is to online > the PMEM as movable zones. If so, and if the low priority jobs are > restricted by cpuset to allocate from PMEM only, we may fail to run > quite some workloads as being discussed in the following threads, > > https://lore.kernel.org/linux-mm/1604470210-124827-1-git-send-email-feng.tang@xxxxxxxxx/ Thanks for sharing the thread. It seems the configuration of movable zone + node bind is not supported very well or need evolve to support new use cases. > > >> > >> Basically I am saying we should put the upfront control (limit) on the > >> usage of top tier memory by the jobs. > > > > This sounds similar to what I talked about in LSFMM 2019 > > (https://lwn.net/Articles/787418/). We used to have some potential > > usecase which divides DRAM:PMEM ratio for different jobs or memcgs > > when I was with Alibaba. > > > > In the first place I thought about per NUMA node limit, but it was > > very hard to configure it correctly for users unless you know exactly > > about your memory usage and hot/cold memory distribution. > > > > I'm wondering, just off the top of my head, if we could extend the > > semantic of low and min limit. For example, just redefine low and min > > to "the limit on top tier memory". Then we could have low priority > > jobs have 0 low/min limit. > > Per my understanding, memory.low/min are for the memory protection > instead of the memory limiting. memory.high is for the memory limiting. Yes, it is not limit. I just misused the term, I actually do mean protection but typed "limit". Sorry for the confusion. > > Best Regards, > Huang, Ying
