[PATCH V6 0/4] mm: frontswap: overview (and proposal to merge at next window) Frontswap is the alter ego of cleancache, the "yang" to cleancache's "yin"... and more precisely frontswap is the provider of anonymous pages to transcendent memory to nicely complement cleancache's providing of clean pagecache pages to transcendent memory. For optimal use of transcendent memory, both are necessary... because a kernel under memory pressure first reclaims clean pagecache pages and, when under more memory pressure, starts swapping anonymous pages. Frontswap and cleancache (which was merged at 3.0) are the only necessary changes to the core kernel for transcendent memory; all other supporting code is implemented as drivers. Frontswap code was first posted publicly in January 2009 and on LKML in May 2009, and has remained very stable for over two years now. It is barely invasive, touching only the swap subsystem and adds only about 100 lines of code to existing swap subsystem code files. It has improved syntactically substantially between V1 and this posting of V6, thanks to the review of a few kernel developers, and has adapted easily to at least one major swap subsystem change. As of 3.1, there are two in-tree users of frontswap patiently waiting for this patchset and for CONFIG_FRONTSWAP to be enabled: zcache (staging driver merged at 2.6.39) and Xen tmem (merged at 3.0 and 3.1). V5 of the frontswap patchset has been in linux-next since next-110603 (and this V6 will be there shortly). Earlier versions of frontswap already appear in some leading-edge distros. I am proposing that frontswap should be merged at the next window and would like to ensure well in advance that any maintainers' issues are resolved. Reviewers new to frontswap/cleancache/transcendent memory are encouraged to read the detailed description and FAQ below. Thanks, Dan Changes since V5: - rebased to 3.1-rc1 - use vzalloc (Bob Liu) - various cautious checks and code clarifications/comments (Konrad Wilk) Changes since V4: - rebased to 3.0-rc1 - fix accidentally posted V4 stale code that failed to compile :-( - more appropriate comments for git commits - change config default to n (even it is runtime-disabled unless registered by an in-kernel user such as zcache or Xen tmem) Changes since V3: - Rebased to 2.6.39 (accomodates minor code movement in swapfile.c) Changes since V2: - Rebased to 2.6.36-rc5 (main change: swap_info is now array of pointers) - Added set/end_page_writeback calls around page unlock on successful put - Changed frontswap_init to hide frontswap_poolid (which is cleancache/tmem oddity that need not be exposed to frontswap) - Document and ensure PageLocked requirements are met (per Andrew Morton feedback in cleancache thread) - Remove incorrect flags set/clear around partial swapoff call in frontswap_shrink - Clarified code testing if frontswap is enabled - Add frontswap_register_ops interface to avoid an unnecessary global (per Christoph Hellwig suggestion in cleancache thread) - Use standard success/fail codes (0/<0) (per Nitin Gupta feedback on cleancache patch) - Added Documentations/vm/frontswap.txt including a FAQ (per Andrew Morton feedback in cleancache thread) - Added Documentation/ABI/testing/sysfs-kernel-mm-frontswap to describe sysfs usage (per Andrew Morton feedback in cleancache thread) - Minor static variable naming cleanup (per Jeremy Fitzhardinge feedback in cleancache thread) Changes since V1: - Rebased to 2.6.34 (no functional changes) - Convert to sane types (per Al Viro comment in cleancache thread) - Define some raw constants (Konrad Wilk) - Performance analysis shows significant advantage for frontswap's synchronous page-at-a-time design (vs batched asynchronous speculated as an alternative design). See http://lkml.org/lkml/2010/5/20/314 This "frontswap" patchset provides a clean API to transcendent memory for swap pages; via this API, frontswap can provide "swap to RAM" functionality for any transcendent memory "driver" such as a Xen tmem, or in-kernel compression via zcache; frontswap also provides a nice interface for swapping to RAM on a remote system (RAMster) and for building pseudo-RAM devices such as on-memory-bus SSD or phase-change memory. A more complete description of frontswap can be found in the introductory comment in Documentation/vm/frontswap.txt (in PATCH 2/4) which is included below for convenience. Note that an earlier version of this patch is now shipping in OpenSuSE 11.2 and will soon ship in a release of Oracle Enterprise Linux. Underlying Xen tmem technology is now shipping in Oracle VM 2.2 and Xen 4.0. Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> Reviewed-by: Jeremy Fitzhardinge <jeremy@xxxxxxxx> Reviewed-by: Konrad Wilk <konrad.wilk@xxxxxxxxxx> Acked-by: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Nitin Gupta <ngupta@xxxxxxxxxx> Cc: Matthew Wilcox <matthew@xxxxxx> Cc: Chris Mason <chris.mason@xxxxxxxxxx> Cc: Jan Beulich <JBeulich@xxxxxxxxxx> Cc: Rik Riel <riel@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Documentation/ABI/testing/sysfs-kernel-mm-frontswap | 16 Documentation/vm/frontswap.txt | 215 ++++++++++++ include/linux/frontswap.h | 92 +++++ include/linux/swap.h | 2 include/linux/swapfile.h | 13 mm/Kconfig | 17 mm/Makefile | 1 mm/frontswap.c | 346 ++++++++++++++++++++ mm/page_io.c | 12 mm/swapfile.c | 62 ++- 10 files changed, 764 insertions(+), 12 deletions(-) (following is a copy of Documentation/vm/frontswap.txt including a FAQ) 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>