Re: [PATCH 0/5] mm: vmscan: fix kswapd writeback regression

["Hillf Danton" <hillf.zj@alibaba-inc.com>]

On January 24, 2017 2:17 AM Johannes Weiner wrote:
> 
> We noticed a regression on multiple hadoop workloads when moving from
> 3.10 to 4.0 and 4.6, which involves kswapd getting tangled up in page
> writeout, causing direct reclaim herds that also don't make progress.
> 
> I tracked it down to the thrash avoidance efforts after 3.10 that make
> the kernel better at keeping use-once cache and use-many cache sorted
> on the inactive and active list, with more aggressive protection of
> the active list as long as there is inactive cache. Unfortunately, our
> workload's use-once cache is mostly from streaming writes. Waiting for
> writes to avoid potential reloads in the future is not a good tradeoff.
> 
> These patches do the following:
> 
> 1. Wake the flushers when kswapd sees a lump of dirty pages. It's
>    possible to be below the dirty background limit and still have
>    cache velocity push them through the LRU. So start a-flushin'.
> 
> 2. Let kswapd only write pages that have been rotated twice. This
>    makes sure we really tried to get all the clean pages on the
>    inactive list before resorting to horrible LRU-order writeback.
> 
> 3. Move rotating dirty pages off the inactive list. Instead of
>    churning or waiting on page writeback, we'll go after clean active
>    cache. This might lead to thrashing, but in this state memory
>    demand outstrips IO speed anyway, and reads are faster than writes.
> 
> More details in the individual changelogs.
> 
>  include/linux/mm_inline.h        |  7 ++++
>  include/linux/mmzone.h           |  2 --
>  include/linux/writeback.h        |  2 +-
>  include/trace/events/writeback.h |  2 +-
>  mm/swap.c                        |  9 ++---
>  mm/vmscan.c                      | 68 +++++++++++++++-----------------------
>  6 files changed, 41 insertions(+), 49 deletions(-)
> 
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>