Re: [PATCH v2 1/1] mm/madvise: add MADV_F_COLLAPSE_LIGHT to process_madvise()

[Michal Hocko]

On Fri 19-01-24 10:03:05, Lance Yang wrote:
> Hey Michal,
> 
> Thanks for taking the time to review!
> 
> On Thu, Jan 18, 2024 at 9:40 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Thu 18-01-24 20:03:46, Lance Yang wrote:
> > [...]
> >
> > before we discuss the semantic, let's focus on the usecase.
> >
> > > Use Cases
> > >
> > > An immediate user of this new functionality is the Go runtime heap allocator
> > > that manages memory in hugepage-sized chunks. In the past, whether it was a
> > > newly allocated chunk through mmap() or a reused chunk released by
> > > madvise(MADV_DONTNEED), the allocator attempted to eagerly back memory with
> > > huge pages using madvise(MADV_HUGEPAGE)[2] and madvise(MADV_COLLAPSE)[3]
> > > respectively. However, both approaches resulted in performance issues; for
> > > both scenarios, there could be entries into direct reclaim and/or compaction,
> > > leading to unpredictable stalls[4]. Now, the allocator can confidently use
> > > process_madvise(MADV_F_COLLAPSE_LIGHT) to attempt the allocation of huge pages.
> >
> > IIUC the primary reason is the cost of the huge page allocation which
> > can be really high if the memory is heavily fragmented and it is called
> > synchronously from the process directly, correct? Can that be worked
> 
> Yes, that's correct.
> 
> > around by process_madvise and performing the operation from a different
> > context? Are there any other reasons to have a different mode?
> 
> In latency-sensitive scenarios, some applications aim to enhance performance
> by utilizing huge pages as much as possible. At the same time, in case of
> allocation failure, they prefer a quick return without triggering direct memory
> reclamation and compaction.

Could you elaborate some more on why?

> > I mean I can think of a more relaxed (opportunistic) MADV_COLLAPSE -
> > e.g. non blocking one to make sure that the caller doesn't really block
> > on resource contention (be it locks or memory availability) because that
> > matches our non-blocking interface in other areas but having a LIGHT
> > operation sounds really vague and the exact semantic would be
> > implementation specific and might change over time. Non-blocking has a
> > clear semantic but it is not really clear whether that is what you
> > really need/want.
> 
> Could you provide me with some suggestions regarding the naming of a
> more relaxed (opportunistic) MADV_COLLAPSE?

Naming is not all that important at this stage (it could be
MADV_COLLAPSE_NOBLOCK for example). The primary question is whether
non-blocking in general is the desired behavior or the implementation
should try but not too hard.

-- 
Michal Hocko
SUSE Labs