diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/frontswap.txt | 59 |
1 files changed, 37 insertions, 22 deletions
diff --git a/Documentation/vm/frontswap.txt b/Documentation/vm/frontswap.txt index c71a019be600..1979f430c1c5 100644 --- a/Documentation/vm/frontswap.txt +++ b/Documentation/vm/frontswap.txt @@ -1,13 +1,20 @@ +.. _frontswap: + +========= +Frontswap +========= + Frontswap provides a "transcendent memory" 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. -(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; all other supporting code -- the "backends" -- is implemented as drivers. -See the LWN.net article "Transcendent memory in a nutshell" for a detailed -overview of frontswap and related kernel parts: -https://lwn.net/Articles/454795/ ) +See the LWN.net article `Transcendent memory in a nutshell`_ +for a detailed overview of frontswap and related kernel parts) + +.. _Transcendent memory in a nutshell: https://lwn.net/Articles/454795/ 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 @@ -50,19 +57,27 @@ or the store fails AND the page is invalidated. This ensures stale data may never be obtained from frontswap. 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: -failed_stores - how many store attempts have failed -loads - how many loads were attempted (all should succeed) -succ_stores - how many store attempts have succeeded -invalidates - how many invalidates were attempted +``failed_stores`` + how many store attempts have failed + +``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. FAQ +=== -1) Where's the value? +* Where's the value? When a workload starts swapping, performance falls through the floor. 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 a memory extension technology. -2) Sure there may be performance advantages in some situations, but - what's the space/time overhead of frontswap? +* 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 @@ -148,8 +163,8 @@ pressure that can potentially outweigh the other advantages. A backend, such as zcache, must implement policies to carefully (but dynamically) manage memory limits to ensure this doesn't happen. -3) OK, how about a quick overview of what this frontswap patch does - in terms that a kernel hacker can grok? +* 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 @@ -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 faster. -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, - or maybe swap-over-nbd/NFS)? +* Can't frontswap be configured as a "special" swap device that is + just higher priority than any real swap device (e.g. like zswap, + or maybe swap-over-nbd/NFS)? No. First, the existing swap subsystem doesn't allow for any kind of 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 some kind of "ghost" swap device and ensure that it is never used. -5) Why this weird definition about "duplicate stores"? If a page - has been previously successfully stored, can't it always be - successfully overwritten? +* Why this weird definition about "duplicate stores"? If a page + has been previously successfully stored, 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 @@ -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, 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 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 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 structures that have, over the years, moved back and forth between |