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Date: Thu, 18 Jan 2024 14:43:23 +0100
From: Michal Hocko <mhocko@suse.com>
To: Lance Yang <ioworker0@gmail.com>
Cc: akpm@linux-foundation.org, zokeefe@google.com, david@redhat.com,
	songmuchun@bytedance.com, shy828301@gmail.com, peterx@redhat.com,
	mknyszek@google.com, minchan@kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [PATCH v2 1/1] mm/madvise: add MADV_F_COLLAPSE_LIGHT to
 process_madvise()
Message-ID: <Zakq-54DFdPu0c2U@tiehlicka>
References: <20240118120347.61817-1-ioworker0@gmail.com>
 <ZakqQyL9t2ffNUIf@tiehlicka>
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Dang, forgot to cc linux-api...

On Thu 18-01-24 14:40:19, Michal Hocko 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
> around by process_madvise and performing the operation from a different
> context? Are there any other reasons to have a different mode?
> 
> 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.
> 
> > [1] https://github.com/torvalds/linux/commit/7d8faaf155454f8798ec56404faca29a82689c77
> > [2] https://github.com/golang/go/commit/8fa9e3beee8b0e6baa7333740996181268b60a3a
> > [3] https://github.com/golang/go/commit/9f9bb26880388c5bead158e9eca3be4b3a9bd2af
> > [4] https://github.com/golang/go/issues/63334
> > 
> > [v1] https://lore.kernel.org/lkml/20240117050217.43610-1-ioworker0@gmail.com/
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs