--- /dev/null
+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.
+
+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 "put_page" will
+copy the page to transcendent memory and associate it with the type and
+offset associated with the page. A "get_page" will copy the page, if found,
+from transcendent memory into kernel memory, but will NOT remove the page
+from from transcendent memory. A "flush_page" will remove the page from
+transcendent memory and a "flush_area" will remove ALL pages associated
+with the swap type (e.g., like swapoff) and notify the "device" to refuse
+further puts with that swap type.
+
+Once a page is successfully put, a matching get 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 put 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 put returns failure, transcendent memory 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 transcendent memory
+(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 frontswap.
+
+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 of pages currently contained in frontswap 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.
+
+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.
+
+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 -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_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
+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)?
+
+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 and to avoid thorny race
+conditions that would unnecessarily and greatly complicate frontswap
+and/or the block I/O subsystem.
+
+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. In zcache, "poorly"
+compressible pages can be rejected, where "poorly" can itself be defined
+dynamically depending on current memory constraints.
+
+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 transcendent memory, 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, last updated August 8, 2011
projects / karo-tx-linux.git / commitdiff