On Wed, Mar 27, 2024 at 03:53:39PM -0700, Yuanchu Xie wrote:
On Wed, Mar 27, 2024 at 2:44 PM Gregory Price gregory.price@memverge.com wrote:
Please note that this proposed interface (move_phys_pages) is very unlikely to be received upstream due to side channel concerns. Instead, it's more likely that the tiering component will expose a "promote X pages from tier A to tier B", and the kernel component would then use/consume hotness information to determine which pages to promote.
I see that mm/memory-tiers.c only has support for demotion. What kind of hotness information do devices typically provide? The OCP proposal is not very specific about this. A list of hot pages with configurable threshold? Access frequency for all pages at configured granularity? Is there a way to tell which NUMA node is accessing them, for page promotion?
(caveat: i'm not a memory-tiers maintainer, you may want to poke at them directly for more information, this is simply spitballing an idea)
I don't know of any public proposals of explicit hotness information provided by hardware yet, just the general proposal.
For the sake of simplicity, I would make the assumption that you have the least information possible - a simple list of "hot addresses" in Host Physcal Address format.
I.e. there's some driver function that amounts to:
uint32_t device_get_hot_addresses(uint64_t *addresses, uint32_t buf_max);
Where the return value is number of addresses the device returned, and the buf_max is the number of addresses that can be read.
Drives providing this functionality would then register this as a callback when its memory becomes a member of some numa node.
Re: source node - Devices have no real way of determining upstream source information.
(Just as one example, there are many more realistic designs)
So if there is a way to expose workingset data to the mm/memory_tiers.c component instead of via sysfs/cgroup - that is preferable.
Appreciate the feedback. The data in its current form might be useful to inform demotion decisions, but for promotion, are you aware of any recent developments? I would like to encode hotness as workingset data as well.
There were some recent patches to DAMON about promotion/demotion. You might look there.
~Gregory