Re: [RFC 0/7] introduce memory hinting API for external process

[Cc linux-api]

On Tue 21-05-19 13:39:50, Minchan Kim wrote:

On Mon, May 20, 2019 at 12:46:05PM -0400, Johannes Weiner wrote:

quoted

On Mon, May 20, 2019 at 12:52:47PM +0900, Minchan Kim wrote:

quoted

- Approach

The approach we chose was to use a new interface to allow userspace to
proactively reclaim entire processes by leveraging platform information.
This allowed us to bypass the inaccuracy of the kernel’s LRUs for pages
that are known to be cold from userspace and to avoid races with lmkd
by reclaiming apps as soon as they entered the cached state. Additionally,
it could provide many chances for platform to use much information to
optimize memory efficiency.

IMHO we should spell it out that this patchset complements MADV_WONTNEED
and MADV_FREE by adding non-destructive ways to gain some free memory
space. MADV_COLD is similar to MADV_WONTNEED in a way that it hints the
kernel that memory region is not currently needed and should be reclaimed
immediately; MADV_COOL is similar to MADV_FREE in a way that it hints the
kernel that memory region is not currently needed and should be reclaimed
when memory pressure rises.

I agree with this approach and the semantics. But these names are very
vague and extremely easy to confuse since they're so similar.

MADV_COLD could be a good name, but for deactivating pages, not
reclaiming them - marking memory "cold" on the LRU for later reclaim.

For the immediate reclaim one, I think there is a better option too:
In virtual memory speak, putting a page into secondary storage (or
ensuring it's already there), and then freeing its in-memory copy, is
called "paging out". And that's what this flag is supposed to do. So
how about MADV_PAGEOUT?

With that, we'd have:

MADV_FREE: Mark data invalid, free memory when needed
MADV_DONTNEED: Mark data invalid, free memory immediately

MADV_COLD: Data is not used for a while, free memory when needed
MADV_PAGEOUT: Data is not used for a while, free memory immediately

What do you think?

There are several suggestions until now. Thanks, Folks!

For deactivating:

- MADV_COOL
- MADV_RECLAIM_LAZY
- MADV_DEACTIVATE
- MADV_COLD
- MADV_FREE_PRESERVE


For reclaiming:

- MADV_COLD
- MADV_RECLAIM_NOW
- MADV_RECLAIMING
- MADV_PAGEOUT
- MADV_DONTNEED_PRESERVE

It seems everybody doesn't like MADV_COLD so want to go with other.
For consisteny of view with other existing hints of madvise, -preserve
postfix suits well. However, originally, I don't like the naming FREE
vs DONTNEED from the beginning. They were easily confused.
I prefer PAGEOUT to RECLAIM since it's more likely to be nuance to
represent reclaim with memory pressure and is supposed to paged-in
if someone need it later. So, it imply PRESERVE.
If there is not strong against it, I want to go with MADV_COLD and
MADV_PAGEOUT.

Other opinion?

I do not really care strongly. I am pretty sure we will have a lot of
suggestions because people tend to be good at arguing about that...
Anyway, unlike DONTNEED/FREE we do not have any other OS to implement
these features, right? So we shouldn't be tight to existing names.
On the other hand I kinda like the reference to the existing names but
DEACTIVATE/PAGEOUT seem a good fit to me as well. Unless there is way
much better name suggested I would go with one of those. Up to you.
-- 
Michal Hocko
SUSE Labs