[Example usage, others are tmem, ramster, and RFC KVM] 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. A nice overview of it is visible at: http://lwn.net/Articles/454795/ The backends, such as zcache provides compression of pages resulting in speed up [https://lkml.org/lkml/2012/3/22/383]. If the backends are not enabled and CONFIG_FRONTSWAP is compiled there is no adverse performance hit (details further down). Patches are also at: git://git.kernel.org/pub/scm/linux/kernel/git/konrad/mm.git stable/frontswap.v15.squashed The last time these patches were posted [https://lkml.org/lkml/2011/10/27/206] the discussion got very technical - and the feeling I got was that: - The API is too simple. The hard decisions (what to do when memory is low and the pages are mlocked, disk is faster than the CPU compression, need some way to shed pages when OOM conditions are close by, which pages to compress) are left to the backends. Adding VM pressure hooks could solve some (if not all) of these issues? Dan is working on figuring this out for zcache. - the backends - like zcache - are tied in how this API is used. This means that to get zcache out of staging need to think of the frontswap and zcache (and also the other backends). So to rehash, I think the core issues were with the backends [note, I ommitted some here - that is not b/c I choose to ignore them - it just that there were some many and I believe many of them got resolved in the discussion?] - IRQ latency of zcache. Code needs to be rewritten in zcache a bit. - VM pressure - need to be able to influence the backends (or the frontswap API as a whole) - backends need to handle batched requests. - backend needs to fix glaring atrocities (casting of 'struct page *' to 'char *' and then back). frontswap API: - perhaps make it a device driver and stack it (similar to loopback)? So frontswap_ioctl /dev/sda3 /dev/frontswap1 [bind it] swapon /dev/frontswap1 The work required here means more invasive patches in the swap code and block code to deal with a dynamic sized disk. - only do it on recently activate pages [https://lkml.org/lkml/2012/1/26/520] Compared to the last posting [https://lkml.org/lkml/2011/10/27/206] - a writethrough option. Meaning hat every swap page gets written to both frontswap AND to the swap disk. The backend can choose to tell frontswap to stop sending pages to it. This would allow the backend to react to memory pressure or to shut down if it is taking too much time compressing and pages end up in proper swap. - change the put/get to a different name: store/load - make the return values be bool instead of the int - it was getting confusing - seperate the CONFIG_DEBUGFS options out Documentation/vm/frontswap.txt | 278 +++++++++++++++++++++++++++++ MAINTAINERS | 7 + drivers/staging/ramster/zcache-main.c | 8 +- drivers/staging/zcache/zcache-main.c | 10 +- drivers/xen/tmem.c | 8 +- include/linux/frontswap.h | 127 +++++++++++++ include/linux/swap.h | 4 + include/linux/swapfile.h | 13 ++ mm/Kconfig | 17 ++ mm/Makefile | 1 + mm/frontswap.c | 314 +++++++++++++++++++++++++++++++++ mm/page_io.c | 12 ++ mm/swapfile.c | 54 +++++-- 13 files changed, 827 insertions(+), 26 deletions(-) Dan Magenheimer (4): mm: frontswap: add frontswap header file mm: frontswap: core swap subsystem hooks and headers mm: frontswap: core frontswap functionality mm: frontswap: config and doc files Konrad Rzeszutek Wilk (2): MAINTAINER: Add myself for the frontswap API frontswap: s/put_page/store/g s/get_page/load Benchmarks: James asked about if compiled in but not used (so FRONTSWAP=y but no 'zcache' on the Linux comnand line argument). Dan did some work in this and found that the that CONFIG_FRONTSWAP=y is faster... or actually realistically just well within any measureable noise. -- 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>