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docs/vm: frontswap.txt: convert to ReST format

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Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mike Rapoport 2018-03-21 21:22:20 +02:00 committed by Jonathan Corbet
parent 5ef829e056
commit 76b387bd3c
1 changed files with 37 additions and 22 deletions
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Documentation/vm/frontswap.txt
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@ -1,13 +1,20 @@
.. _frontswap:
=========
Frontswap
=========
Frontswap provides a "transcendent memory" interface for swap pages. Frontswap provides a "transcendent memory" interface for swap pages.
In some environments, dramatic performance savings may be obtained because 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. swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk.
(Note, frontswap -- and cleancache (merged at 3.0) -- are the "frontends" (Note, frontswap -- and :ref:`cleancache` (merged at 3.0) -- are the "frontends"
and the only necessary changes to the core kernel for transcendent memory; and the only necessary changes to the core kernel for transcendent memory;
all other supporting code -- the "backends" -- is implemented as drivers. all other supporting code -- the "backends" -- is implemented as drivers.
See the LWN.net article "Transcendent memory in a nutshell" for a detailed See the LWN.net article `Transcendent memory in a nutshell`_
overview of frontswap and related kernel parts: for a detailed overview of frontswap and related kernel parts)
https://lwn.net/Articles/454795/ )
.. _Transcendent memory in a nutshell: https://lwn.net/Articles/454795/
Frontswap is so named because it can be thought of as the opposite of 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 "backing" store for a swap device. The storage is assumed to be
@ -50,19 +57,27 @@ or the store fails AND the page is invalidated. This ensures stale data may
never be obtained from frontswap. never be obtained from frontswap.
If properly configured, monitoring of frontswap is done via debugfs in If properly configured, monitoring of frontswap is done via debugfs in
the /sys/kernel/debug/frontswap directory. The effectiveness of the `/sys/kernel/debug/frontswap` directory. The effectiveness of
frontswap can be measured (across all swap devices) with: frontswap can be measured (across all swap devices) with:
failed_stores - how many store attempts have failed ``failed_stores``
loads - how many loads were attempted (all should succeed) how many store attempts have failed
succ_stores - how many store attempts have succeeded
invalidates - how many invalidates were attempted ``loads``
how many loads were attempted (all should succeed)
``succ_stores``
how many store attempts have succeeded
``invalidates``
how many invalidates were attempted
A backend implementation may provide additional metrics. A backend implementation may provide additional metrics.
FAQ FAQ
===
1) Where's the value? * Where's the value?
When a workload starts swapping, performance falls through the floor. When a workload starts swapping, performance falls through the floor.
Frontswap significantly increases performance in many such workloads by Frontswap significantly increases performance in many such workloads by
@ -117,8 +132,8 @@ A KVM implementation is underway and has been RFC'ed to lkml. And,
using frontswap, investigation is also underway on the use of NVM as using frontswap, investigation is also underway on the use of NVM as
a memory extension technology. a memory extension technology.
2) Sure there may be performance advantages in some situations, but * Sure there may be performance advantages in some situations, but
what's the space/time overhead of frontswap? what's the space/time overhead of frontswap?
If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into
nothingness and the only overhead is a few extra bytes per swapon'ed nothingness and the only overhead is a few extra bytes per swapon'ed
@ -148,8 +163,8 @@ pressure that can potentially outweigh the other advantages. A
backend, such as zcache, must implement policies to carefully (but backend, such as zcache, must implement policies to carefully (but
dynamically) manage memory limits to ensure this doesn't happen. dynamically) manage memory limits to ensure this doesn't happen.
3) OK, how about a quick overview of what this frontswap patch does * OK, how about a quick overview of what this frontswap patch does
in terms that a kernel hacker can grok? in terms that a kernel hacker can grok?
Let's assume that a frontswap "backend" has registered during Let's assume that a frontswap "backend" has registered during
kernel initialization; this registration indicates that this kernel initialization; this registration indicates that this
@ -188,9 +203,9 @@ and (potentially) a swap device write are replaced by a "frontswap backend
store" and (possibly) a "frontswap backend loads", which are presumably much store" and (possibly) a "frontswap backend loads", which are presumably much
faster. faster.
4) Can't frontswap be configured as a "special" swap device that is * Can't frontswap be configured as a "special" swap device that is
just higher priority than any real swap device (e.g. like zswap, just higher priority than any real swap device (e.g. like zswap,
or maybe swap-over-nbd/NFS)? or maybe swap-over-nbd/NFS)?
No. First, the existing swap subsystem doesn't allow for any kind of No. First, the existing swap subsystem doesn't allow for any kind of
swap hierarchy. Perhaps it could be rewritten to accommodate a hierarchy, swap hierarchy. Perhaps it could be rewritten to accommodate a hierarchy,
@ -240,9 +255,9 @@ installation, frontswap is useless. Swapless portable devices
can still use frontswap but a backend for such devices must configure can still use frontswap but a backend for such devices must configure
some kind of "ghost" swap device and ensure that it is never used. some kind of "ghost" swap device and ensure that it is never used.
5) Why this weird definition about "duplicate stores"? If a page * Why this weird definition about "duplicate stores"? If a page
has been previously successfully stored, can't it always be has been previously successfully stored, can't it always be
successfully overwritten? successfully overwritten?
Nearly always it can, but no, sometimes it cannot. Consider an example Nearly always it can, but no, sometimes it cannot. Consider an example
where data is compressed and the original 4K page has been compressed where data is compressed and the original 4K page has been compressed
@ -254,7 +269,7 @@ 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, swap subsystem then writes the new data to the read swap device,
this is the correct course of action to ensure coherency. this is the correct course of action to ensure coherency.
6) What is frontswap_shrink for? * What is frontswap_shrink for?
When the (non-frontswap) swap subsystem swaps out a page to a real 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 swap device, that page is only taking up low-value pre-allocated disk
@ -267,7 +282,7 @@ to "repatriate" pages sent to a remote machine back to the local machine;
this is driven using the frontswap_shrink mechanism when memory pressure this is driven using the frontswap_shrink mechanism when memory pressure
subsides. subsides.
7) Why does the frontswap patch create the new include file swapfile.h? * Why does the frontswap patch create the new include file swapfile.h?
The frontswap code depends on some swap-subsystem-internal data The frontswap code depends on some swap-subsystem-internal data
structures that have, over the years, moved back and forth between structures that have, over the years, moved back and forth between