Re: [PATCH v3 0/7] mm: demotion: Introduce new node state N_DEMOTION_TARGETS

ying.huang
2022-04-26T07:56:12Z

On Mon, 2022-04-25 at 16:45 +0530, Jagdish Gediya wrote:
> On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@intel.com wrote:
> > On Sat, 2022-04-23 at 01:25 +0530, Jagdish Gediya wrote:
> > > Some systems(e.g. PowerVM) can have both DRAM(fast memory) only
> > > NUMA node which are N_MEMORY and slow memory(persistent memory)
> > > only NUMA node which are also N_MEMORY. As the current demotion
> > > target finding algorithm works based on N_MEMORY and best distance,
> > > it will choose DRAM only NUMA node as demotion target instead of
> > > persistent memory node on such systems. If DRAM only NUMA node is
> > > filled with demoted pages then at some point new allocations can
> > > start falling to persistent memory, so basically cold pages are in
> > > fast memor (due to demotion) and new pages are in slow memory, this
> > > is why persistent memory nodes should be utilized for demotion and
> > > dram node should be avoided for demotion so that they can be used
> > > for new allocations.
> > >
> > > Current implementation can work fine on the system where the memory
> > > only numa nodes are possible only for persistent/slow memory but it
> > > is not suitable for the like of systems mentioned above.
> >
> > Can you share the NUMA topology information of your machine? And the
> > demotion order before and after your change?
> >
> > Whether it's good to use the PMEM nodes as the demotion targets of the
> > DRAM-only node too?
>
> $ numactl -H
> available: 2 nodes (0-1)
> node 0 cpus: 0 1 2 3 4 5 6 7
> node 0 size: 14272 MB
> node 0 free: 13392 MB
> node 1 cpus:
> node 1 size: 2028 MB
> node 1 free: 1971 MB
> node distances:
> node 0 1
>   0: 10 40
>   1: 40 10
>
> 1) without N_DEMOTION_TARGETS patch series, 1 is demotion target
>    for 0 even when 1 is DRAM node and there is no demotion targets for 1.
>
> $ cat /sys/bus/nd/devices/dax0.0/target_node
> 2
> $
> # cd /sys/bus/dax/drivers/
> :/sys/bus/dax/drivers# ls
> device_dax kmem
> :/sys/bus/dax/drivers# cd device_dax/
> :/sys/bus/dax/drivers/device_dax# echo dax0.0 > unbind
> :/sys/bus/dax/drivers/device_dax# echo dax0.0 > ../kmem/new_id
> :/sys/bus/dax/drivers/device_dax# numactl -H
> available: 3 nodes (0-2)
> node 0 cpus: 0 1 2 3 4 5 6 7
> node 0 size: 14272 MB
> node 0 free: 13380 MB
> node 1 cpus:
> node 1 size: 2028 MB
> node 1 free: 1961 MB
> node 2 cpus:
> node 2 size: 0 MB
> node 2 free: 0 MB
> node distances:
> node 0 1 2
>   0: 10 40 80
>   1: 40 10 80
>   2: 80 80 10
>

This looks like a virtual machine, not a real machine. That's
unfortunate. I am looking forward to a real issue, not a theoritical
possible issue.

> 2) Once this new node brought online, without N_DEMOTION_TARGETS
> patch series, 1 is demotion target for 0 and 2 is demotion target
> for 1.
>
> With this patch series applied,
> 1) No demotion target for either 0 or 1 before dax device is online
> 2) 2 is demotion target for both 0 and 1 after dax device is online.
>

So with your change, if a node hasn't N_DEMOTION_TARGETS, it will become
a top-level demotion source even if it hasn't N_CPU? If so, I cannot
clear N_DEMOTION_TARGETS for a node in middle or bottom level?

Best Regards,
Huang, Ying

> >
[snip]