On Thu, Feb 02, 2017 at 02:14:10PM +0900, Minchan Kim wrote:

Hi Johannes,

On Tue, Jan 31, 2017 at 04:38:10PM -0500, Johannes Weiner wrote:

quoted

On Tue, Jan 31, 2017 at 11:45:47AM -0800, Shaohua Li wrote:

quoted

On Tue, Jan 31, 2017 at 01:59:49PM -0500, Johannes Weiner wrote:

quoted

Hi Shaohua,

On Sun, Jan 29, 2017 at 09:51:17PM -0800, Shaohua Li wrote:

quoted

We are trying to use MADV_FREE in jemalloc. Several issues are found. Without
solving the issues, jemalloc can't use the MADV_FREE feature.
- Doesn't support system without swap enabled. Because if swap is off, we can't
  or can't efficiently age anonymous pages. And since MADV_FREE pages are mixed
  with other anonymous pages, we can't reclaim MADV_FREE pages. In current
  implementation, MADV_FREE will fallback to MADV_DONTNEED without swap enabled.
  But in our environment, a lot of machines don't enable swap. This will prevent
  our setup using MADV_FREE.
- Increases memory pressure. page reclaim bias file pages reclaim against
  anonymous pages. This doesn't make sense for MADV_FREE pages, because those
  pages could be freed easily and refilled with very slight penality. Even page
  reclaim doesn't bias file pages, there is still an issue, because MADV_FREE
  pages and other anonymous pages are mixed together. To reclaim a MADV_FREE
  page, we probably must scan a lot of other anonymous pages, which is
  inefficient. In our test, we usually see oom with MADV_FREE enabled and nothing
  without it.

Fully agreed, the anon LRU is a bad place for these pages.

quoted

For the first two issues, introducing a new LRU list for MADV_FREE pages could
solve the issues. We can directly reclaim MADV_FREE pages without writting them
out to swap, so the first issue could be fixed. If only MADV_FREE pages are in
the new list, page reclaim can easily reclaim such pages without interference
of file or anonymous pages. The memory pressure issue will disappear.

Do we actually need a new page flag and a special LRU for them? These
pages are basically like clean cache pages at that point. What do you
think about clearing their PG_swapbacked flag on MADV_FREE and moving
them to the inactive file list? The way isolate+putback works should
not even need much modification, something like clear_page_mlock().

When the reclaim scanner finds anon && dirty && !swapbacked, it can
again set PG_swapbacked and goto keep_locked to move the page back
into the anon LRU to get reclaimed according to swapping rules.

Interesting idea! Not sure though, the MADV_FREE pages are actually anonymous
pages, this will introduce confusion. On the other hand, if the MADV_FREE pages
are mixed with inactive file pages, page reclaim need to reclaim a lot of file
pages first before reclaim the MADV_FREE pages. This doesn't look good. The
point of a separate LRU is to avoid scan other anon/file pages.

The LRU code and the rest of VM already use independent page type
distinctions. That's because shmem pages are !PageAnon - they have a
page->mapping that points to a real address space, not an anon_vma -
but they are swapbacked and thus go through the anon LRU. This would
just do the reverse: put PageAnon pages on the file LRU when they
don't contain valid data and are thus not swapbacked.

As far as mixing with inactive file pages goes, it'd be possible to
link the MADV_FREE pages to the tail of the inactive list, rather than
the head. That said, I'm not sure reclaiming use-once filesystem cache
before MADV_FREE is such a bad policy. MADV_FREE retains the vmas for
the sole purpose of reusing them in the (near) future. That is
actually a stronger reuse signal than we have for use-once file pages.
If somebody does continuous writes to a logfile or a one-off search
through one or more files, we should actually reclaim that cache
before we go after MADV_FREE pages that are temporarily invalidated.

Yes, we should be careful on this issue. It was main arguable point.
How about moving them to head of inactive file, not tail if we want to
go with inactive file LRU?

With that, VM try to reclaim file pages first from the tail of list
and if pages reclaimed were workingset, it could be activated by
workingset_refault. Otherwise, we can discard use-once pages without
puring *madv_free* pages so I think it's good compromise.

What do you think?

That's what I tried to say. To address Shaohua's concern in two steps,
first, it *would* be possible to move MADV_FREE pages to the tail of
the inactive list. But then, taking a step back, I argued that this is
probably not be the reclaim policy we actually want.

So I agree with you. I think MADV_FREE should move these pages to the
*head* of the inactive cache list, so that we reclaim colder use-once
cache first. Workingset detection will make any necessary corrections.

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