Re: [RFC PATCH v1 00/11] Manage the top tier memory in a tiered memory

[Yang Shi]

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