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