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-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"
-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/ )
-
-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" conforming
-to the requirements of transcendent memory (such as Xen's "tmem", or
-in-kernel compressed memory, aka "zcache", or future RAM-like devices);
-this pseudo-RAM device is not directly accessible or addressable by the
-kernel and is of unknown and possibly time-varying size. The driver
-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 device to receive frontswap pages associated
-with the specified swap device number (aka "type"). A "store" will
-copy the page to transcendent memory and associate it with the type and
-offset associated with the page. A "load" will copy the page, if found,
-from transcendent memory into kernel memory, but will NOT remove the page
-from from transcendent memory. An "invalidate_page" will remove the page
-from transcendent memory and an "invalidate_area" will remove ALL pages
-associated with the swap type (e.g., like swapoff) and notify the "device"
-to refuse further stores with that swap type.
-
-Once a page is successfully stored, a matching load on the page will normally
-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 store returns
-success, the data has been successfully saved to transcendent memory and
-a disk write and, if the data is later read back, a disk read are avoided.
-If a store returns failure, transcendent memory has rejected the data, and the
-page can be written to swap as usual.
-
-If a backend chooses, frontswap can be configured as a "writethrough
-cache" by calling frontswap_writethrough(). In this mode, the reduction
-in swap device writes is lost (and also a non-trivial performance advantage)
-in order to allow the backend to arbitrarily "reclaim" space used to
-store frontswap pages to more completely manage its memory usage.
-
-Note that if a page is stored and the page already exists in transcendent memory
-(a "duplicate" store), either the store succeeds and the data is overwritten,
-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
-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
-
-A backend implementation may provide additional metrics.
-
-FAQ
-
-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
-"transcendent memory" 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 device" and the frontswap (and
-cleancache) interface to transcendent memory provides a nice way to read
-and write -- and indirectly "name" -- the pages.
-
-Frontswap -- and cleancache -- with a fairly small impact on the kernel,
-provides a huge amount of flexibility for more dynamic, flexible RAM
-utilization in various system configurations:
-
-In the single kernel case, aka "zcache", pages are compressed and
-stored in local memory, thus increasing the total anonymous pages
-that can be safely kept in RAM. Zcache essentially trades off CPU
-cycles used in compression/decompression for better memory utilization.
-Benchmarks have shown little or no impact when memory pressure is
-low while providing a significant performance improvement (25%+)
-on some workloads under high memory pressure.
-
-"RAMster" builds on zcache by adding "peer-to-peer" transcendent memory
-support for clustered systems. Frontswap pages are locally compressed
-as in zcache, but then "remotified" to another system's RAM. This
-allows RAM to be dynamically load-balanced back-and-forth as needed,
-i.e. when system A is overcommitted, it can swap to system B, and
-vice versa. RAMster can also be configured as a memory server so
-many servers in a cluster can swap, dynamically as needed, to a single
-server configured with a large amount of RAM... without pre-configuring
-how much of the RAM is available for each of the clients!
-
-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).
-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 "selfballooning"), 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), thus mitigating the potentially awful performance impact
-of unplanned swapping.
-
-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?
-
-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 "store"
-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.
-
-When swap pages are stored in transcendent memory instead of written
-out to disk, there is a side effect that this may create more memory
-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?
-
-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 "store" 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_store is called. Frontswap
-consults with the frontswap backend and if the backend says it does NOT
-have room, frontswap_store returns -1 and the kernel swaps the page
-to the swap device as normal. Note that the response from the frontswap
-backend is unpredictable to the kernel; 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_load() 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
-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)?
-
-No. First, the existing swap subsystem doesn't allow for any kind of
-swap hierarchy. Perhaps it could be rewritten to accomodate a hierarchy,
-but this would require fairly drastic changes. Even if it were
-rewritten, the existing swap subsystem uses the block I/O layer which
-assumes a swap device is fixed size and any page in it is linearly
-addressable. Frontswap barely touches the existing swap subsystem,
-and works around the constraints of the block I/O subsystem to provide
-a great deal of flexibility and dynamicity.
-
-For example, the acceptance of any swap page by the frontswap backend is
-entirely unpredictable. This is critical to the definition of frontswap
-backends because it grants completely dynamic discretion to the
-backend. In zcache, one cannot know a priori how compressible a page is.
-"Poorly" compressible pages can be rejected, and "poorly" can itself be
-defined dynamically depending on current memory constraints.
-
-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 and to avoid thorny race
-conditions that would unnecessarily and greatly complicate frontswap
-and/or the block I/O subsystem. That said, only the initial "store"
-and "load" operations need be synchronous. A separate asynchronous thread
-is free to manipulate the pages stored by frontswap. For example,
-the "remotification" thread in RAMster uses standard asynchronous
-kernel sockets to move compressed frontswap pages to a remote machine.
-Similarly, a KVM guest-side implementation could do in-guest compression
-and use "batched" hypercalls.
-
-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.
-
-There is a downside to the transcendent memory specifications for
-frontswap: Since any "store" 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. This 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. 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?
-
-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 store must be rejected. Whenever
-frontswap rejects a store that would overwrite, it also must invalidate
-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 transcendent memory, that
-page may be taking up valuable real estate. The frontswap_shrink
-routine allows code outside of the swap subsystem to force pages out
-of the memory managed by frontswap and back into kernel-addressable memory.
-For example, in RAMster, a "suction driver" thread will attempt
-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?
-
-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, last updated April 9, 2012