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linux/mm/Kconfig
Huang Ying 38d8b4e6bd mm, THP, swap: delay splitting THP during swap out 
Patch series "THP swap: Delay splitting THP during swapping out", v11.

This patchset is to optimize the performance of Transparent Huge Page
(THP) swap.

Recently, the performance of the storage devices improved so fast that
we cannot saturate the disk bandwidth with single logical CPU when do
page swap out even on a high-end server machine.  Because the
performance of the storage device improved faster than that of single
logical CPU.  And it seems that the trend will not change in the near
future.  On the other hand, the THP becomes more and more popular
because of increased memory size.  So it becomes necessary to optimize
THP swap performance.

The advantages of the THP swap support include:

 - Batch the swap operations for the THP to reduce lock
   acquiring/releasing, including allocating/freeing the swap space,
   adding/deleting to/from the swap cache, and writing/reading the swap
   space, etc. This will help improve the performance of the THP swap.

 - The THP swap space read/write will be 2M sequential IO. It is
   particularly helpful for the swap read, which are usually 4k random
   IO. This will improve the performance of the THP swap too.

 - It will help the memory fragmentation, especially when the THP is
   heavily used by the applications. The 2M continuous pages will be
   free up after THP swapping out.

 - It will improve the THP utilization on the system with the swap
   turned on. Because the speed for khugepaged to collapse the normal
   pages into the THP is quite slow. After the THP is split during the
   swapping out, it will take quite long time for the normal pages to
   collapse back into the THP after being swapped in. The high THP
   utilization helps the efficiency of the page based memory management
   too.

There are some concerns regarding THP swap in, mainly because possible
enlarged read/write IO size (for swap in/out) may put more overhead on
the storage device.  To deal with that, the THP swap in should be turned
on only when necessary.  For example, it can be selected via
"always/never/madvise" logic, to be turned on globally, turned off
globally, or turned on only for VMA with MADV_HUGEPAGE, etc.

This patchset is the first step for the THP swap support.  The plan is
to delay splitting THP step by step, finally avoid splitting THP during
the THP swapping out and swap out/in the THP as a whole.

As the first step, in this patchset, the splitting huge page is delayed
from almost the first step of swapping out to after allocating the swap
space for the THP and adding the THP into the swap cache.  This will
reduce lock acquiring/releasing for the locks used for the swap cache
management.

With the patchset, the swap out throughput improves 15.5% (from about
3.73GB/s to about 4.31GB/s) in the vm-scalability swap-w-seq test case
with 8 processes.  The test is done on a Xeon E5 v3 system.  The swap
device used is a RAM simulated PMEM (persistent memory) device.  To test
the sequential swapping out, the test case creates 8 processes, which
sequentially allocate and write to the anonymous pages until the RAM and
part of the swap device is used up.

This patch (of 5):

In this patch, splitting huge page is delayed from almost the first step
of swapping out to after allocating the swap space for the THP
(Transparent Huge Page) and adding the THP into the swap cache.  This
will batch the corresponding operation, thus improve THP swap out
throughput.

This is the first step for the THP swap optimization.  The plan is to
delay splitting the THP step by step and avoid splitting the THP
finally.

In this patch, one swap cluster is used to hold the contents of each THP
swapped out.  So, the size of the swap cluster is changed to that of the
THP (Transparent Huge Page) on x86_64 architecture (512).  For other
architectures which want such THP swap optimization,
ARCH_USES_THP_SWAP_CLUSTER needs to be selected in the Kconfig file for
the architecture.  In effect, this will enlarge swap cluster size by 2
times on x86_64.  Which may make it harder to find a free cluster when
the swap space becomes fragmented.  So that, this may reduce the
continuous swap space allocation and sequential write in theory.  The
performance test in 0day shows no regressions caused by this.

In the future of THP swap optimization, some information of the swapped
out THP (such as compound map count) will be recorded in the
swap_cluster_info data structure.

The mem cgroup swap accounting functions are enhanced to support charge
or uncharge a swap cluster backing a THP as a whole.

The swap cluster allocate/free functions are added to allocate/free a
swap cluster for a THP.  A fair simple algorithm is used for swap
cluster allocation, that is, only the first swap device in priority list
will be tried to allocate the swap cluster.  The function will fail if
the trying is not successful, and the caller will fallback to allocate a
single swap slot instead.  This works good enough for normal cases.  If
the difference of the number of the free swap clusters among multiple
swap devices is significant, it is possible that some THPs are split
earlier than necessary.  For example, this could be caused by big size
difference among multiple swap devices.

The swap cache functions is enhanced to support add/delete THP to/from
the swap cache as a set of (HPAGE_PMD_NR) sub-pages.  This may be
enhanced in the future with multi-order radix tree.  But because we will
split the THP soon during swapping out, that optimization doesn't make
much sense for this first step.

The THP splitting functions are enhanced to support to split THP in swap
cache during swapping out.  The page lock will be held during allocating
the swap cluster, adding the THP into the swap cache and splitting the
THP.  So in the code path other than swapping out, if the THP need to be
split, the PageSwapCache(THP) will be always false.

The swap cluster is only available for SSD, so the THP swap optimization
in this patchset has no effect for HDD.

[ying.huang@intel.com: fix two issues in THP optimize patch]
  Link: http://lkml.kernel.org/r/87k25ed8zo.fsf@yhuang-dev.intel.com
[hannes@cmpxchg.org: extensive cleanups and simplifications, reduce code size]
Link: http://lkml.kernel.org/r/20170515112522.32457-2-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Andrew Morton <akpm@linux-foundation.org> [for config option]
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> [for changes in huge_memory.c and huge_mm.h]
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-07-06 16:24:31 -07:00

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config SELECT_MEMORY_MODEL
def_bool y
depends on ARCH_SELECT_MEMORY_MODEL
choice
prompt "Memory model"
depends on SELECT_MEMORY_MODEL
default DISCONTIGMEM_MANUAL if ARCH_DISCONTIGMEM_DEFAULT
default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT
default FLATMEM_MANUAL
config FLATMEM_MANUAL
bool "Flat Memory"
depends on !(ARCH_DISCONTIGMEM_ENABLE || ARCH_SPARSEMEM_ENABLE) || ARCH_FLATMEM_ENABLE
help
This option allows you to change some of the ways that
Linux manages its memory internally. Most users will
only have one option here: FLATMEM. This is normal
and a correct option.
Some users of more advanced features like NUMA and
memory hotplug may have different options here.
DISCONTIGMEM is a more mature, better tested system,
but is incompatible with memory hotplug and may suffer
decreased performance over SPARSEMEM. If unsure between
"Sparse Memory" and "Discontiguous Memory", choose
"Discontiguous Memory".
If unsure, choose this option (Flat Memory) over any other.
config DISCONTIGMEM_MANUAL
bool "Discontiguous Memory"
depends on ARCH_DISCONTIGMEM_ENABLE
help
This option provides enhanced support for discontiguous
memory systems, over FLATMEM. These systems have holes
in their physical address spaces, and this option provides
more efficient handling of these holes. However, the vast
majority of hardware has quite flat address spaces, and
can have degraded performance from the extra overhead that
this option imposes.
Many NUMA configurations will have this as the only option.
If unsure, choose "Flat Memory" over this option.
config SPARSEMEM_MANUAL
bool "Sparse Memory"
depends on ARCH_SPARSEMEM_ENABLE
help
This will be the only option for some systems, including
memory hotplug systems. This is normal.
For many other systems, this will be an alternative to
"Discontiguous Memory". This option provides some potential
performance benefits, along with decreased code complexity,
but it is newer, and more experimental.
If unsure, choose "Discontiguous Memory" or "Flat Memory"
over this option.
endchoice
config DISCONTIGMEM
def_bool y
depends on (!SELECT_MEMORY_MODEL && ARCH_DISCONTIGMEM_ENABLE) || DISCONTIGMEM_MANUAL
config SPARSEMEM
def_bool y
depends on (!SELECT_MEMORY_MODEL && ARCH_SPARSEMEM_ENABLE) || SPARSEMEM_MANUAL
config FLATMEM
def_bool y
depends on (!DISCONTIGMEM && !SPARSEMEM) || FLATMEM_MANUAL
config FLAT_NODE_MEM_MAP
def_bool y
depends on !SPARSEMEM
#
# Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
# to represent different areas of memory. This variable allows
# those dependencies to exist individually.
#
config NEED_MULTIPLE_NODES
def_bool y
depends on DISCONTIGMEM || NUMA
config HAVE_MEMORY_PRESENT
def_bool y
depends on ARCH_HAVE_MEMORY_PRESENT || SPARSEMEM
#
# SPARSEMEM_EXTREME (which is the default) does some bootmem
# allocations when memory_present() is called. If this cannot
# be done on your architecture, select this option. However,
# statically allocating the mem_section[] array can potentially
# consume vast quantities of .bss, so be careful.
#
# This option will also potentially produce smaller runtime code
# with gcc 3.4 and later.
#
config SPARSEMEM_STATIC
bool
#
# Architecture platforms which require a two level mem_section in SPARSEMEM
# must select this option. This is usually for architecture platforms with
# an extremely sparse physical address space.
#
config SPARSEMEM_EXTREME
def_bool y
depends on SPARSEMEM && !SPARSEMEM_STATIC
config SPARSEMEM_VMEMMAP_ENABLE
bool
config SPARSEMEM_ALLOC_MEM_MAP_TOGETHER
def_bool y
depends on SPARSEMEM && X86_64
config SPARSEMEM_VMEMMAP
bool "Sparse Memory virtual memmap"
depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
default y
help
SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
pfn_to_page and page_to_pfn operations. This is the most
efficient option when sufficient kernel resources are available.
config HAVE_MEMBLOCK
bool
config HAVE_MEMBLOCK_NODE_MAP
bool
config HAVE_MEMBLOCK_PHYS_MAP
bool
config HAVE_GENERIC_GUP
bool
config ARCH_DISCARD_MEMBLOCK
bool
config NO_BOOTMEM
bool
config MEMORY_ISOLATION
bool
config MOVABLE_NODE
bool "Enable to assign a node which has only movable memory"
depends on HAVE_MEMBLOCK
depends on NO_BOOTMEM
depends on X86_64 || OF_EARLY_FLATTREE || MEMORY_HOTPLUG
depends on NUMA
default n
help
Allow a node to have only movable memory. Pages used by the kernel,
such as direct mapping pages cannot be migrated. So the corresponding
memory device cannot be hotplugged. This option allows the following
two things:
- When the system is booting, node full of hotpluggable memory can
be arranged to have only movable memory so that the whole node can
be hot-removed. (need movable_node boot option specified).
- After the system is up, the option allows users to online all the
memory of a node as movable memory so that the whole node can be
hot-removed.
Users who don't use the memory hotplug feature are fine with this
option on since they don't specify movable_node boot option or they
don't online memory as movable.
Say Y here if you want to hotplug a whole node.
Say N here if you want kernel to use memory on all nodes evenly.
#
# Only be set on architectures that have completely implemented memory hotplug
# feature. If you are not sure, don't touch it.
#
config HAVE_BOOTMEM_INFO_NODE
def_bool n
# eventually, we can have this option just 'select SPARSEMEM'
config MEMORY_HOTPLUG
bool "Allow for memory hot-add"
depends on SPARSEMEM || X86_64_ACPI_NUMA
depends on ARCH_ENABLE_MEMORY_HOTPLUG
depends on COMPILE_TEST || !KASAN
config MEMORY_HOTPLUG_SPARSE
def_bool y
depends on SPARSEMEM && MEMORY_HOTPLUG
config MEMORY_HOTPLUG_DEFAULT_ONLINE
bool "Online the newly added memory blocks by default"
default n
depends on MEMORY_HOTPLUG
help
This option sets the default policy setting for memory hotplug
onlining policy (/sys/devices/system/memory/auto_online_blocks) which
determines what happens to newly added memory regions. Policy setting
can always be changed at runtime.
See Documentation/memory-hotplug.txt for more information.
Say Y here if you want all hot-plugged memory blocks to appear in
'online' state by default.
Say N here if you want the default policy to keep all hot-plugged
memory blocks in 'offline' state.
config MEMORY_HOTREMOVE
bool "Allow for memory hot remove"
select MEMORY_ISOLATION
select HAVE_BOOTMEM_INFO_NODE if (X86_64 || PPC64)
depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
depends on MIGRATION
# Heavily threaded applications may benefit from splitting the mm-wide
# page_table_lock, so that faults on different parts of the user address
# space can be handled with less contention: split it at this NR_CPUS.
# Default to 4 for wider testing, though 8 might be more appropriate.
# ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
# PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
# DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page.
#
config SPLIT_PTLOCK_CPUS
int
default "999999" if !MMU
default "999999" if ARM && !CPU_CACHE_VIPT
default "999999" if PARISC && !PA20
default "4"
config ARCH_ENABLE_SPLIT_PMD_PTLOCK
bool
#
# support for memory balloon
config MEMORY_BALLOON
bool
#
# support for memory balloon compaction
config BALLOON_COMPACTION
bool "Allow for balloon memory compaction/migration"
def_bool y
depends on COMPACTION && MEMORY_BALLOON
help
Memory fragmentation introduced by ballooning might reduce
significantly the number of 2MB contiguous memory blocks that can be
used within a guest, thus imposing performance penalties associated
with the reduced number of transparent huge pages that could be used
by the guest workload. Allowing the compaction & migration for memory
pages enlisted as being part of memory balloon devices avoids the
scenario aforementioned and helps improving memory defragmentation.
#
# support for memory compaction
config COMPACTION
bool "Allow for memory compaction"
def_bool y
select MIGRATION
depends on MMU
help
Compaction is the only memory management component to form
high order (larger physically contiguous) memory blocks
reliably. The page allocator relies on compaction heavily and
the lack of the feature can lead to unexpected OOM killer
invocations for high order memory requests. You shouldn't
disable this option unless there really is a strong reason for
it and then we would be really interested to hear about that at
linux-mm@kvack.org.
#
# support for page migration
#
config MIGRATION
bool "Page migration"
def_bool y
depends on (NUMA || ARCH_ENABLE_MEMORY_HOTREMOVE || COMPACTION || CMA) && MMU
help
Allows the migration of the physical location of pages of processes
while the virtual addresses are not changed. This is useful in
two situations. The first is on NUMA systems to put pages nearer
to the processors accessing. The second is when allocating huge
pages as migration can relocate pages to satisfy a huge page
allocation instead of reclaiming.
config ARCH_ENABLE_HUGEPAGE_MIGRATION
bool
config PHYS_ADDR_T_64BIT
def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT
config BOUNCE
bool "Enable bounce buffers"
default y
depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
help
Enable bounce buffers for devices that cannot access
the full range of memory available to the CPU. Enabled
by default when ZONE_DMA or HIGHMEM is selected, but you
may say n to override this.
# On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
# have more than 4GB of memory, but we don't currently use the IOTLB to present
# a 32-bit address to OHCI. So we need to use a bounce pool instead.
config NEED_BOUNCE_POOL
bool
default y if TILE && USB_OHCI_HCD
config NR_QUICK
int
depends on QUICKLIST
default "1"
config VIRT_TO_BUS
bool
help
An architecture should select this if it implements the
deprecated interface virt_to_bus(). All new architectures
should probably not select this.
config MMU_NOTIFIER
bool
select SRCU
config KSM
bool "Enable KSM for page merging"
depends on MMU
help
Enable Kernel Samepage Merging: KSM periodically scans those areas
of an application's address space that an app has advised may be
mergeable. When it finds pages of identical content, it replaces
the many instances by a single page with that content, so
saving memory until one or another app needs to modify the content.
Recommended for use with KVM, or with other duplicative applications.
See Documentation/vm/ksm.txt for more information: KSM is inactive
until a program has madvised that an area is MADV_MERGEABLE, and
root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).
config DEFAULT_MMAP_MIN_ADDR
int "Low address space to protect from user allocation"
depends on MMU
default 4096
help
This is the portion of low virtual memory which should be protected
from userspace allocation. Keeping a user from writing to low pages
can help reduce the impact of kernel NULL pointer bugs.
For most ia64, ppc64 and x86 users with lots of address space
a value of 65536 is reasonable and should cause no problems.
On arm and other archs it should not be higher than 32768.
Programs which use vm86 functionality or have some need to map
this low address space will need CAP_SYS_RAWIO or disable this
protection by setting the value to 0.
This value can be changed after boot using the
/proc/sys/vm/mmap_min_addr tunable.
config ARCH_SUPPORTS_MEMORY_FAILURE
bool
config MEMORY_FAILURE
depends on MMU
depends on ARCH_SUPPORTS_MEMORY_FAILURE
bool "Enable recovery from hardware memory errors"
select MEMORY_ISOLATION
select RAS
help
Enables code to recover from some memory failures on systems
with MCA recovery. This allows a system to continue running
even when some of its memory has uncorrected errors. This requires
special hardware support and typically ECC memory.
config HWPOISON_INJECT
tristate "HWPoison pages injector"
depends on MEMORY_FAILURE && DEBUG_KERNEL && PROC_FS
select PROC_PAGE_MONITOR
config NOMMU_INITIAL_TRIM_EXCESS
int "Turn on mmap() excess space trimming before booting"
depends on !MMU
default 1
help
The NOMMU mmap() frequently needs to allocate large contiguous chunks
of memory on which to store mappings, but it can only ask the system
allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently
more than it requires. To deal with this, mmap() is able to trim off
the excess and return it to the allocator.
If trimming is enabled, the excess is trimmed off and returned to the
system allocator, which can cause extra fragmentation, particularly
if there are a lot of transient processes.
If trimming is disabled, the excess is kept, but not used, which for
long-term mappings means that the space is wasted.
Trimming can be dynamically controlled through a sysctl option
(/proc/sys/vm/nr_trim_pages) which specifies the minimum number of
excess pages there must be before trimming should occur, or zero if
no trimming is to occur.
This option specifies the initial value of this option. The default
of 1 says that all excess pages should be trimmed.
See Documentation/nommu-mmap.txt for more information.
config TRANSPARENT_HUGEPAGE
bool "Transparent Hugepage Support"
depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
select COMPACTION
select RADIX_TREE_MULTIORDER
help
Transparent Hugepages allows the kernel to use huge pages and
huge tlb transparently to the applications whenever possible.
This feature can improve computing performance to certain
applications by speeding up page faults during memory
allocation, by reducing the number of tlb misses and by speeding
up the pagetable walking.
If memory constrained on embedded, you may want to say N.
choice
prompt "Transparent Hugepage Support sysfs defaults"
depends on TRANSPARENT_HUGEPAGE
default TRANSPARENT_HUGEPAGE_ALWAYS
help
Selects the sysfs defaults for Transparent Hugepage Support.
config TRANSPARENT_HUGEPAGE_ALWAYS
bool "always"
help
Enabling Transparent Hugepage always, can increase the
memory footprint of applications without a guaranteed
benefit but it will work automatically for all applications.
config TRANSPARENT_HUGEPAGE_MADVISE
bool "madvise"
help
Enabling Transparent Hugepage madvise, will only provide a
performance improvement benefit to the applications using
madvise(MADV_HUGEPAGE) but it won't risk to increase the
memory footprint of applications without a guaranteed
benefit.
endchoice
config ARCH_WANTS_THP_SWAP
def_bool n
config THP_SWAP
def_bool y
depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP
help
Swap transparent huge pages in one piece, without splitting.
XXX: For now this only does clustered swap space allocation.
For selection by architectures with reasonable THP sizes.
config TRANSPARENT_HUGE_PAGECACHE
def_bool y
depends on TRANSPARENT_HUGEPAGE
#
# UP and nommu archs use km based percpu allocator
#
config NEED_PER_CPU_KM
depends on !SMP
bool
default y
config CLEANCACHE
bool "Enable cleancache driver to cache clean pages if tmem is present"
default n
help
Cleancache can be thought of as a page-granularity victim cache
for clean pages that the kernel's pageframe replacement algorithm
(PFRA) would like to keep around, but can't since there isn't enough
memory. So when the PFRA "evicts" a page, it first attempts to use
cleancache code to put the data contained in that page into
"transcendent memory", memory that is not directly accessible or
addressable by the kernel and is of unknown and possibly
time-varying size. And when a cleancache-enabled
filesystem wishes to access a page in a file on disk, it first
checks cleancache to see if it already contains it; if it does,
the page is copied into the kernel and a disk access is avoided.
When a transcendent memory driver is available (such as zcache or
Xen transcendent memory), a significant I/O reduction
may be achieved. When none is available, all cleancache calls
are reduced to a single pointer-compare-against-NULL resulting
in a negligible performance hit.
If unsure, say Y to enable cleancache
config FRONTSWAP
bool "Enable frontswap to cache swap pages if tmem is present"
depends on SWAP
default n
help
Frontswap is so named because it can be thought of as the opposite
of a "backing" store for a swap device. The data is stored into
"transcendent memory", memory that is not directly accessible or
addressable by the kernel and is of unknown and possibly
time-varying size. When space in transcendent memory is available,
a significant swap I/O reduction may be achieved. When none is
available, all frontswap calls are reduced to a single pointer-
compare-against-NULL resulting in a negligible performance hit
and swap data is stored as normal on the matching swap device.
If unsure, say Y to enable frontswap.
config CMA
bool "Contiguous Memory Allocator"
depends on HAVE_MEMBLOCK && MMU
select MIGRATION
select MEMORY_ISOLATION
help
This enables the Contiguous Memory Allocator which allows other
subsystems to allocate big physically-contiguous blocks of memory.
CMA reserves a region of memory and allows only movable pages to
be allocated from it. This way, the kernel can use the memory for
pagecache and when a subsystem requests for contiguous area, the
allocated pages are migrated away to serve the contiguous request.
If unsure, say "n".
config CMA_DEBUG
bool "CMA debug messages (DEVELOPMENT)"
depends on DEBUG_KERNEL && CMA
help
Turns on debug messages in CMA. This produces KERN_DEBUG
messages for every CMA call as well as various messages while
processing calls such as dma_alloc_from_contiguous().
This option does not affect warning and error messages.
config CMA_DEBUGFS
bool "CMA debugfs interface"
depends on CMA && DEBUG_FS
help
Turns on the DebugFS interface for CMA.
config CMA_AREAS
int "Maximum count of the CMA areas"
depends on CMA
default 7
help
CMA allows to create CMA areas for particular purpose, mainly,
used as device private area. This parameter sets the maximum
number of CMA area in the system.
If unsure, leave the default value "7".
config MEM_SOFT_DIRTY
bool "Track memory changes"
depends on CHECKPOINT_RESTORE && HAVE_ARCH_SOFT_DIRTY && PROC_FS
select PROC_PAGE_MONITOR
help
This option enables memory changes tracking by introducing a
soft-dirty bit on pte-s. This bit it set when someone writes
into a page just as regular dirty bit, but unlike the latter
it can be cleared by hands.
See Documentation/vm/soft-dirty.txt for more details.
config ZSWAP
bool "Compressed cache for swap pages (EXPERIMENTAL)"
depends on FRONTSWAP && CRYPTO=y
select CRYPTO_LZO
select ZPOOL
default n
help
A lightweight compressed cache for swap pages. It takes
pages that are in the process of being swapped out and attempts to
compress them into a dynamically allocated RAM-based memory pool.
This can result in a significant I/O reduction on swap device and,
in the case where decompressing from RAM is faster that swap device
reads, can also improve workload performance.
This is marked experimental because it is a new feature (as of
v3.11) that interacts heavily with memory reclaim. While these
interactions don't cause any known issues on simple memory setups,
they have not be fully explored on the large set of potential
configurations and workloads that exist.
config ZPOOL
tristate "Common API for compressed memory storage"
default n
help
Compressed memory storage API. This allows using either zbud or
zsmalloc.
config ZBUD
tristate "Low (Up to 2x) density storage for compressed pages"
default n
help
A special purpose allocator for storing compressed pages.
It is designed to store up to two compressed pages per physical
page. While this design limits storage density, it has simple and
deterministic reclaim properties that make it preferable to a higher
density approach when reclaim will be used.
config Z3FOLD
tristate "Up to 3x density storage for compressed pages"
depends on ZPOOL
default n
help
A special purpose allocator for storing compressed pages.
It is designed to store up to three compressed pages per physical
page. It is a ZBUD derivative so the simplicity and determinism are
still there.
config ZSMALLOC
tristate "Memory allocator for compressed pages"
depends on MMU
default n
help
zsmalloc is a slab-based memory allocator designed to store
compressed RAM pages. zsmalloc uses virtual memory mapping
in order to reduce fragmentation. However, this results in a
non-standard allocator interface where a handle, not a pointer, is
returned by an alloc(). This handle must be mapped in order to
access the allocated space.
config PGTABLE_MAPPING
bool "Use page table mapping to access object in zsmalloc"
depends on ZSMALLOC
help
By default, zsmalloc uses a copy-based object mapping method to
access allocations that span two pages. However, if a particular
architecture (ex, ARM) performs VM mapping faster than copying,
then you should select this. This causes zsmalloc to use page table
mapping rather than copying for object mapping.
You can check speed with zsmalloc benchmark:
https://github.com/spartacus06/zsmapbench
config ZSMALLOC_STAT
bool "Export zsmalloc statistics"
depends on ZSMALLOC
select DEBUG_FS
help
This option enables code in the zsmalloc to collect various
statistics about whats happening in zsmalloc and exports that
information to userspace via debugfs.
If unsure, say N.
config GENERIC_EARLY_IOREMAP
bool
config MAX_STACK_SIZE_MB
int "Maximum user stack size for 32-bit processes (MB)"
default 80
range 8 256 if METAG
range 8 2048
depends on STACK_GROWSUP && (!64BIT || COMPAT)
help
This is the maximum stack size in Megabytes in the VM layout of 32-bit
user processes when the stack grows upwards (currently only on parisc
and metag arch). The stack will be located at the highest memory
address minus the given value, unless the RLIMIT_STACK hard limit is
changed to a smaller value in which case that is used.
A sane initial value is 80 MB.
# For architectures that support deferred memory initialisation
config ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
bool
config DEFERRED_STRUCT_PAGE_INIT
bool "Defer initialisation of struct pages to kthreads"
default n
depends on ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
depends on NO_BOOTMEM && MEMORY_HOTPLUG
depends on !FLATMEM
help
Ordinarily all struct pages are initialised during early boot in a
single thread. On very large machines this can take a considerable
amount of time. If this option is set, large machines will bring up
a subset of memmap at boot and then initialise the rest in parallel
by starting one-off "pgdatinitX" kernel thread for each node X. This
has a potential performance impact on processes running early in the
lifetime of the system until these kthreads finish the
initialisation.
config IDLE_PAGE_TRACKING
bool "Enable idle page tracking"
depends on SYSFS && MMU
select PAGE_EXTENSION if !64BIT
help
This feature allows to estimate the amount of user pages that have
not been touched during a given period of time. This information can
be useful to tune memory cgroup limits and/or for job placement
within a compute cluster.
See Documentation/vm/idle_page_tracking.txt for more details.
config ZONE_DEVICE
bool "Device memory (pmem, etc...) hotplug support"
depends on MEMORY_HOTPLUG
depends on MEMORY_HOTREMOVE
depends on SPARSEMEM_VMEMMAP
depends on X86_64 #arch_add_memory() comprehends device memory
help
Device memory hotplug support allows for establishing pmem,
or other device driver discovered memory regions, in the
memmap. This allows pfn_to_page() lookups of otherwise
"device-physical" addresses which is needed for using a DAX
mapping in an O_DIRECT operation, among other things.
If FS_DAX is enabled, then say Y.
config FRAME_VECTOR
bool
config ARCH_USES_HIGH_VMA_FLAGS
bool
config ARCH_HAS_PKEYS
bool
config PERCPU_STATS
bool "Collect percpu memory statistics"
default n
help
This feature collects and exposes statistics via debugfs. The
information includes global and per chunk statistics, which can
be used to help understand percpu memory usage.
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