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Provide unmapped page cache control (v2)

From:  Balbir Singh <>
Subject:  [PATCH 0/3] Provide unmapped page cache control (v2)
Date:  Fri, 10 Dec 2010 19:59:20 +0530
Message-ID:  <20101210142745.29934.29186.stgit@localhost6.localdomain6>
Archive-link:  Article

The following series implements page cache control,
this is a split out version of patch 1 of version 3 of the
page cache optimization patches posted earlier at
Previous posting

The previous revision received lot of comments, I've tried to
address as many of those as possible in this revision. The
last series was reviewed-by Christoph Lameter.

There were comments on overlap with Nick's changes and overlap
with them. I don't feel these changes impact Nick's work and
integration can/will be considered as the patches evolve, if
need be.

Detailed Description
This patch implements unmapped page cache control via preferred
page cache reclaim. The current patch hooks into kswapd and reclaims
page cache if the user has requested for unmapped page control.
This is useful in the following scenario
- In a virtualized environment with cache=writethrough, we see
  double caching - (one in the host and one in the guest). As
  we try to scale guests, cache usage across the system grows.
  The goal of this patch is to reclaim page cache when Linux is running
  as a guest and get the host to hold the page cache and manage it.
  There might be temporary duplication, but in the long run, memory
  in the guests would be used for mapped pages.
- The option is controlled via a boot option and the administrator
  can selectively turn it on, on a need to use basis.

A lot of the code is borrowed from zone_reclaim_mode logic for
__zone_reclaim(). One might argue that the with ballooning and
KSM this feature is not very useful, but even with ballooning,
we need extra logic to balloon multiple VM machines and it is hard
to figure out the correct amount of memory to balloon. With these
patches applied, each guest has a sufficient amount of free memory
available, that can be easily seen and reclaimed by the balloon driver.
The additional memory in the guest can be reused for additional
applications or used to start additional guests/balance memory in
the host.

KSM currently does not de-duplicate host and guest page cache. The goal
of this patch is to help automatically balance unmapped page cache when
instructed to do so.

There are some magic numbers in use in the code, UNMAPPED_PAGE_RATIO
and the number of pages to reclaim when unmapped_page_control argument
is supplied. These numbers were chosen to avoid aggressiveness in
reaping page cache ever so frequently, at the same time providing control.

The sysctl for min_unmapped_ratio provides further control from
within the guest on the amount of unmapped pages to reclaim.

Data from the previous patchsets can be found at

Size measurement

# size mm/built-in.o 
   text    data     bss     dec     hex filename
 419431 1883047  140888 2443366  254866 mm/built-in.o

# size mm/built-in.o 
   text    data     bss     dec     hex filename
 418908 1883023  140888 2442819  254643 mm/built-in.o


Balbir Singh (3):
      Move zone_reclaim() outside of CONFIG_NUMA
      Refactor zone_reclaim, move reusable functionality outside
      Provide control over unmapped pages

 Documentation/kernel-parameters.txt |    8 ++
 include/linux/mmzone.h              |    4 +
 include/linux/swap.h                |   21 +++++-
 init/Kconfig                        |   12 +++
 kernel/sysctl.c                     |   20 +++--
 mm/page_alloc.c                     |    9 ++
 mm/vmscan.c                         |  132 +++++++++++++++++++++++++++++++----
 7 files changed, 175 insertions(+), 31 deletions(-)

Three Cheers,
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