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

[Minchan Kim]

On Mon, May 20, 2019 at 12:46:05PM -0400, Johannes Weiner wrote:
> On Mon, May 20, 2019 at 12:52:47PM +0900, Minchan Kim wrote:
> > - 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 kernelâs 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?