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Subject[PATCH V3 0/4] Frontswap: overview
[PATCH V3 0/4] Frontswap: overview

Changes since V2:
- Rebased to 2.6.36-rc5 (main change: swap_info is now array of pointers)
- Added set/end_page_writeback calls around page unlock on successful put
- Changed frontswap_init to hide frontswap_poolid (which is cleancache/tmem
oddity that need not be exposed to frontswap)
- Document and ensure PageLocked requirements are met (per Andrew Morton
feedback in cleancache thread)
- Remove incorrect flags set/clear around partial swapoff call in
- Clarified code testing if frontswap is enabled
- Add frontswap_register_ops interface to avoid an unnecessary global (per
Christoph Hellwig suggestion in cleancache thread)
- Use standard success/fail codes (0/<0) (per Nitin Gupta feedback on
cleancache patch)
- Added Documentations/vm/frontswap.txt including a FAQ (per Andrew Morton
feedback in cleancache thread)
- Added Documentation/ABI/testing/sysfs-kernel-mm-frontswap to describe
sysfs usage (per Andrew Morton feedback in cleancache thread)
- Minor static variable naming cleanup (per Jeremy Fitzhardinge feedback
in cleancache thread)

Changes since V1:
- Rebased to 2.6.34 (no functional changes)
- Convert to sane types (per Al Viro comment in cleancache thread)
- Define some raw constants (Konrad Wilk)
- Performance analysis shows significant advantage for frontswap's
synchronous page-at-a-time design (vs batched asynchronous speculated
as an alternative design). See

In previous patch postings, frontswap was part of the Transcendent
Memory ("tmem") patchset. This patchset refocuses not on the underlying
technology (tmem) but instead on the useful functionality provided for Linux,
and provides a clean API so that frontswap can provide this very useful
functionality via a Xen tmem driver OR completely independent of tmem.
For example: an in-kernel compression "backend" for frontswap can be
implemented and some believe frontswap will be a very nice interface
for building RAM-like functionality for pseudo-RAM devices such as
on-memory-bus SSD or phase-change memory.

A more complete description of frontswap can be found in the introductory
comment in Documentation/vm/frontswap.txt (in PATCH 2/4) which is included
below for convenience.

Note that an earlier version of this patch is now shipping in OpenSuSE 11.2
and will soon ship in a release of Oracle Enterprise Linux. Underlying
tmem technology is now shipping in Oracle VM 2.2 and Xen 4.0.

Signed-off-by: Dan Magenheimer <>
Reviewed-by: Jeremy Fitzhardinge <>

Documentation/ABI/testing/sysfs-kernel-mm-frontswap | 16
Documentation/vm/frontswap.txt | 209 ++++++++++++
include/linux/frontswap.h | 86 +++++
include/linux/swap.h | 2
include/linux/swapfile.h | 13
mm/Kconfig | 17 +
mm/Makefile | 1
mm/frontswap.c | 331 ++++++++++++++++++++
mm/page_io.c | 12
mm/swapfile.c | 58 ++-
10 files changed, 736 insertions(+), 9 deletions(-)

(following is a copy of Documentation/vm/frontswap.txt including a FAQ)

Frontswap provides a page-accessible-memory (PAM) interface for swap pages.
In some environments, dramatic performance savings may be obtained because
swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk.

Frontswap is so named because it can be thought of as the opposite of
a "backing" store for a swap device. The storage is assumed to be
a synchronous concurrency-safe page-oriented pseudo-RAM device (such as
Xen's Transcendent Memory, aka "tmem", or in-kernel compressed memory,
aka "zmem", or other RAM-like devices) which is not directly accessible
or addressable by the kernel and is of unknown and possibly time-varying
size. This pseudo-RAM device links itself to frontswap by calling
frontswap_register_ops to set the frontswap_ops funcs appropriately and
the functions it provides must conform to certain policies as follows:

An "init" prepares the pseudo-RAM to receive frontswap pages associated
with the specified swap device number (aka "type"). A "put_page" will
copy the page to pseudo-RAM and associate it with the type and offset
associated with the page. A "get_page" will copy the page, if found,
from pseudo-RAM into kernel memory, but will NOT remove the page from
pseudo-RAM. A "flush_page" will remove the page from pseudo-RAM and a
"flush_area" will remove ALL pages associated with the swap type
(e.g., like swapoff) and notify the pseudo-RAM device to refuse
further puts with that swap type.

Once a page is successfully put, a matching get on the page will always
succeed. So when the kernel finds itself in a situation where it needs
to swap out a page, it first attempts to use frontswap. If the put returns
non-zero, the data has been successfully saved to pseudo-RAM and
a disk write and, if the data is later read back, a disk read are avoided.
If a put returns zero, pseudo-RAM has rejected the data, and the page can
be written to swap as usual.

Note that if a page is put and the page already exists in pseudo-RAM
(a "duplicate" put), either the put succeeds and the data is overwritten,
or the put fails AND the page is flushed. This ensures stale data may
never be obtained from psuedo-RAM.

Monitoring and control of frontswap is done by sysfs files in the
/sys/kernel/mm/frontswap directory. The effectiveness of frontswap can
be measured (across all swap devices) with:

curr_pages - number of pages currently contained in frontswap
failed_puts - how many put attempts have failed
gets - how many gets were attempted (all should succeed)
succ_puts - how many put attempts have succeeded
flushes - how many flushes were attempted

The number can be reduced by root by writing an integer target to curr_pages,
which results in a "partial swapoff", thus reducing the number of frontswap
pages to that target if memory constraints permit.


1) Where's the value?

When a workload starts swapping, performance falls through the floor.
Frontswap significantly increases performance in many such workloads by
providing a clean, dynamic interface to read and write swap pages to
pseudo-RAM -- RAM that is otherwise not directly addressable to the kernel.
This interface is ideal when data is transformed to a different form
and size (such as with compression) or secretly moved (as might be
useful for write-balancing for some RAM-like devices). Swap pages (and
evicted page-cache pages) are a great use for this kind of slower-than-RAM-
but-much-faster-than-disk pseudo-RAM and the frontswap (and cleancache)
"page-object-oriented" specification provides a nice way to read
and write -- and indirectly "name" -- the pages.

In the virtual case, the whole point of virtualization is to statistically
multiplex physical resources acrosst the varying demands of multiple
virtual machines. This is really hard to do with RAM and efforts to do
it well with no kernel changes have essentially failed (except in some
well-publicized special-case workloads). Frontswap -- and cleancache --
with a fairly small impact on the kernel, provides a huge amount
of flexibility for more dynamic, flexible RAM multiplexing.
Specifically, the Xen Transcendent Memory backend allows otherwise
"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
virtual machines, but the pages can be compressed and deduplicated to
optimize RAM utilization. And when guest OS's are induced to surrender
underutilized RAM (e.g. with "self-ballooning"), sudden unexpected
memory pressure may result in swapping; frontswap allows those pages
to be swapped to and from hypervisor RAM if overall host system memory
conditions allow.

2) Sure there may be performance advantages in some situations, but
what's the space/time overhead of frontswap?

If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into
nothingness and the only overhead is a few extra bytes per swapon'ed
swap device. If CONFIG_FRONTSWAP is enabled but no frontswap "backend"
registers, there is one extra global variable compared to zero for
every swap page read or written. If CONFIG_FRONTSWAP is enabled
AND a frontswap backend registers AND the backend fails every "put"
request (i.e. provides no memory despite claiming it might),
CPU overhead is still negligible -- and since every frontswap fail
precedes a swap page write-to-disk, the system is highly likely
to be I/O bound and using a small fraction of a percent of a CPU
will be irrelevant anyway.

As for space, if CONFIG_FRONTSWAP is enabled AND a frontswap backend
registers, one bit is allocated for every swap page for every swap
device that is swapon'd. This is added to the EIGHT bits (which
was sixteen until about 2.6.34) that the kernel already allocates
for every swap page for every swap device that is swapon'd. (Hugh
Dickins has observed that frontswap could probably steal one of
the existing eight bits, but let's worry about that minor optimization
later.) For very large swap disks (which are rare) on a standard
4K pagesize, this is 1MB per 32GB swap.

3) OK, how about a quick overview of what this frontswap patch does
in terms that a kernel hacker can grok?

Let's assume that a frontswap "backend" has registered during
kernel initialization; this registration indicates that this
frontswap backend has access to some "memory" that is not directly
accessible by the kernel. Exactly how much memory it provides is
entirely dynamic and random.

Whenever a swap-device is swapon'd frontswap_init() is called,
passing the swap device number (aka "type") as a parameter.
This notifies frontswap to expect attempts to "put" swap pages
associated with that number.

Whenever the swap subsystem is readying a page to write to a swap
device (c.f swap_writepage()), frontswap_put_page is called. Frontswap
consults with the frontswap backend and if the backend says
it does NOT have room, frontswap_put_page returns 0 and the page is
swapped as normal. Note that the response from the frontswap
backend is essentially random; it may choose to never accept a
page, it could accept every ninth page, or it might accept every
page. But if the backend does accept a page, the data from the page
has already been copied and associated with the type and offset,
and the backend guarantees the persistence of the data. In this case,
frontswap sets a bit in the "frontswap_map" for the swap device
corresponding to the page offset on the swap device to which it would
otherwise have written the data.

When the swap subsystem needs to swap-in a page (swap_readpage()),
it first calls frontswap_get_page() which checks the frontswap_map to
see if the page was earlier accepted by the frontswap backend. If
it was, the page of data is filled from the frontswap backend and
the swap-in is complete. If not, the normal swap-in code is
executed to obtain the page of data from the real swap device.

So every time the frontswap backend accepts a page, a swap device read
and (potentially) a swap device write are replaced by a "frontswap backend
put" and (possibly) a "frontswap backend get", which are presumably much

4) Can't frontswap be configured as a "special" swap device that is
just higher priority than any real swap device (e.g. like zswap)?

No. Recall that acceptance of any swap page by the frontswap
backend is entirely unpredictable. This is critical to the definition
of frontswap because it grants completely dynamic discretion to the
backend. But since any "put" might fail, there must always be a real
slot on a real swap device to swap the page. Thus frontswap must be
implemented as a "shadow" to every swapon'd device with the potential
capability of holding every page that the swap device might have held
and the possibility that it might hold no pages at all.
On the downside, this also means that frontswap cannot contain more
pages than the total of swapon'd swap devices. For example, if NO
swap device is configured on some installation, frontswap is useless.

Further, frontswap is entirely synchronous whereas a real swap
device is, by definition, asynchronous and uses block I/O. The
block I/O layer is not only unnecessary, but may perform "optimizations"
that are inappropriate for a RAM-oriented device including delaying
the write of some pages for a significant amount of time.
Synchrony is required to ensure the dynamicity of the backend.

In a virtualized environment, the dynamicity allows the hypervisor
(or host OS) to do "intelligent overcommit". For example, it can
choose to accept pages only until host-swapping might be imminent,
then force guests to do their own swapping.

5) Why this weird definition about "duplicate puts"? If a page
has been previously successfully put, can't it always be
successfully overwritten?

Nearly always it can, but no, sometimes it cannot. Consider an example
where data is compressed and the original 4K page has been compressed
to 1K. Now an attempt is made to overwrite the page with data that
is non-compressible and so would take the entire 4K. But the backend
has no more space. In this case, the put must be rejected. Whenever
frontswap rejects a put that would overwrite, it also must flush
the old data and ensure that it is no longer accessible. Since the
swap subsystem then writes the new data to the read swap device,
this is the correct course of action to ensure coherency.

6) What is frontswap_shrink for?

When the (non-frontswap) swap subsystem swaps out a page to a real
swap device, that page is only taking up low-value pre-allocated disk
space. But if frontswap has placed a page in pseudo-RAM, that
page may be taking up valuable real estate. The frontswap_shrink
routine allows a process outside of the swap subsystem (such as
a userland service via the sysfs interface, or a kernel thread)
to force pages out of the memory managed by frontswap and back into
kernel-addressable memory.

7) Why does the frontswap patch create the new include file swapfile.h?

The frontswap code depends on some swap-subsystem-internal data
structures that have, over the years, moved back and forth between
static and global. This seemed a reasonable compromise: Define
them as global but declare them in a new include file that isn't
included by the large number of source files that include swap.h.

Dan Magenheimer, September 21 2010

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