Re: RFC: Memory Tiering Kernel Interfaces

[Alistair Popple]

Dave Hansen <dave.hansen@xxxxxxxxx> writes:

> On 5/3/22 10:14, Alistair Popple wrote:
>> I would certainly be interested in figuring out how HW could provide some sort
>> of heatmap to identify which pages are hot and which processing unit is using
>> them. Currently for these systems users have to manually assign memory policy to
>> get any reasonable performance, both to disable NUMA balancing and make sure
>> memory is allocated on the right node.
>
> Autonuma-induced page faults are a total non-starter for lots of
> workloads, even ignoring GPUs.  Basically anyone who is latency
> sensitive stays far, far away from autonuma.
>
> As for improving on page faults for data collection...
>
> *Can* hardware provide this information?  Definitely.
>
> Have hardware vendors been motivated enough to add hardware to do this?
>  Nope, not yet.

Not entirely true. The GPUs on POWER9 have performance counters capable of
collecting this kind of information for memory accessed from the GPU. I will
admit though that sadly most people probably don't have a P9 sitting under their
desk :)

For various reasons these counters weren't exposed to the kernel but that's
something I would like to work on fixing.

> Do you know anyone that works for any hardware companies? ;)

Maybe ;)

> Seriously, though.  Folks at Intel _are_ thinking about this problem.
> I'm hoping we have hardware some day to help lend a hand.  The more
> hardware vendors that do this, the more likely it is that we'll have
> good kernel code to consume data from the hardware.

Agreed.