This still needs testing. I've tried multiple reproduction scenarios locally
but two things are tripping me. One, Simon's network card is using GFP_ATOMIC
allocations where as the one I use locally does not. Second, Simon's is a real
mail workload with network traffic and there are no decent mail simulator
benchmarks (that I could find at least) that would replicate the situation.
Still, I'm hopeful it'll stop kswapd going mad on his machine and might
also alleviate some of the "too much free memory" problem.
Changelog since V1
o Take classzone into account
o Ensure that kswapd always balances at order-09
o Reset classzone and order after reading
o Require a percentage of a node be balanced for high-order allocations,
not just any zone as ZONE_DMA could be balanced when the node in general
is a mess
Simon Kirby reported the following problem
We're seeing cases on a number of servers where cache never fully
grows to use all available memory. Sometimes we see servers with 4
GB of memory that never seem to have less than 1.5 GB free, even with
a constantly-active VM. In some cases, these servers also swap out
while this happens, even though they are constantly reading the working
set into memory. We have been seeing this happening for a long time;
I don't think it's anything recent, and it still happens on 2.6.36.
After some debugging work by Simon, Dave Hansen and others, the prevaling
theory became that kswapd is reclaiming order-3 pages requested by SLUB
too aggressive about it.
There are two apparent problems here. On the target machine, there is a small
Normal zone in comparison to DMA32. As kswapd tries to balance all zones, it
would continually try reclaiming for Normal even though DMA32 was balanced
enough for callers. The second problem is that sleeping_prematurely() uses
the requested order, not the order kswapd finally reclaimed at. This keeps
kswapd artifically awake.
This series aims to alleviate these problems but needs testing to confirm
it alleviates the actual problem and wider review to think if there is a
better alternative approach. Local tests passed but are not reproducing
the same problem unfortunately so the results are inclusive.
include/linux/mmzone.h | 3 +-
mm/page_alloc.c | 8 ++-
mm/vmscan.c | 109 ++++++++++++++++++++++++++++++++++++++++-------
3 files changed, 99 insertions(+), 21 deletions(-)
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