Re: [RFC] mm: add new syscall pidfd_set_mempolicy()

[Frank van der Linden]

On Wed, Oct 12, 2022 at 5:34 AM Vinicius Petrucci <vpetrucci@xxxxxxxxx> wrote:
>
> > Well, per address range operation is a completely different beast I
> > would say. External tool would need to a) understand what that range is
> > used for (e.g. stack/heap ranges, mmaped shared files like libraries or
> > private mappings) and b) by in sync with memory layout modifications
> > done by applications (e.g. that an mmap has been issued to back malloc
> > request). Quite a lot of understanding about the specific process. I
> > would say that with that intimate knowledge it is quite better to be
> > part of the process and do those changes from within of the process
> > itself.
>
> Sorry, this may be a digression, but just wanted to mention a
> particular use case from a project I recently collaborated on (to
> appear next month at IIWSC 2022:
> http://www.iiswc.org/iiswc2022/index.html).
>
> We carried out a performance analysis of the latest Linux AutoNUMA
> memory tiering on graph processing applications. We noticed that hot
> pages cannot be properly identified by the reactive approach used by
> AutoNUMA due to irregular/random memory access patterns. Thus, as a
> POC, we implemented and evaluated a simple idea of having an external
> user-level process/agent that, based on prior profiling results of
> memory regions, could make more effectively memory chunk/object-based
> mappings (instead of page-level allocation/migration) in advance on
> either DRAM or CXL/PMEM (via mbind calls). This kind of tiering
> solution could deliver up to 2x more performance for graph analytics
> workloads. We plan to evaluate other workloads as well.
>
> Having a feature like "pidfd/process_mbind" would really simplify our
> user-level agent implementation moving forward, as right now we are
> adding a LD_PRELOAD wrapper (for signal handler) to listen and execute
> "mbind" requests from another process. If there's any other
> alternative solution to this already (via ptrace?), please let me
> know.
>

Interesting, looking forward to seeing your paper! This is the kind of
use case I was trying to describe for pidfd_mbind() - a userspace
orchestrator with some intimate knowledge of the process' memory
layout (through profiling, like in your case, or otherwise), that can
direct memory to the right nodes / memory tiers.

- Frank