Re: [RFC] net: use atomic allocation for order-3 page allocation

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It's somewhat an intractable problem to know if compaction will succeed
without trying it, and you can certainly end up in a state where memory is
heavily fragmented, even with compaction running. You can't compact kernel
pages for example, so you can end up in a state where compaction does
nothing through no fault of it's own.

In this case you waste time in compaction routines, then end up reclaiming
precious page cache pages or swapping out for whatever it is your machine
was doing trying to do to satisfy these order-3 allocations, after which all
those pages need to be restored from disk almost immediately. This is not a
happy server. Any mm fix may be years away. The only simple solution I can
think of is specifically caching these allocations, in any other case under
memory pressure they will be split by other smaller allocations.

We've been forcing these allocations to order-0 internally until we can
think of something else.

-Deb

On Thu, Jun 11, 2015 at 4:48 PM, Eric Dumazet [off-list ref]
wrote:

quoted

On Thu, 2015-06-11 at 13:24 -0700, Shaohua Li wrote:

quoted

We saw excessive memory compaction triggered by skb_page_frag_refill.
This causes performance issues. Commit 5640f7685831e0 introduces the
order-3 allocation to improve performance. But memory compaction has
high overhead. The benefit of order-3 allocation can't compensate the
overhead of memory compaction.

This patch makes the order-3 page allocation atomic. If there is no
memory pressure and memory isn't fragmented, the alloction will still
success, so we don't sacrifice the order-3 benefit here. If the atomic
allocation fails, compaction will not be triggered and we will fallback
to order-0 immediately.

The mellanox driver does similar thing, if this is accepted, we must fix
the driver too.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Shaohua Li <redacted>
---
 net/core/sock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 292f422..e9855a4 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -1883,7 +1883,7 @@ bool skb_page_frag_refill(unsigned int sz, struct

page_frag *pfrag, gfp_t gfp)

      pfrag->offset = 0;
      if (SKB_FRAG_PAGE_ORDER) {
-             pfrag->page = alloc_pages(gfp | __GFP_COMP |
+             pfrag->page = alloc_pages((gfp & ~__GFP_WAIT) | __GFP_COMP
|
                                        __GFP_NOWARN | __GFP_NORETRY,
                                        SKB_FRAG_PAGE_ORDER);
              if (likely(pfrag->page)) {

This is not a specific networking issue, but mm one.

You really need to start a discussion with mm experts.

Your changelog does not exactly explains what _is_ the problem.

If the problem lies in mm layer, it might be time to fix it, instead of
work around the bug by never triggering it from this particular point,
which is a safe point where a process is willing to wait a bit.

Memory compaction is either working as intending, or not.

If we enabled it but never run it because it hurts, what is the point
enabling it ?



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