Patch #1 sets up some helper functions for accounting. Patch #2 adds writeback visibility in /proc/sys/vm. To help developers and applications gain visibility into writeback behaviour adding two read-only sysctl files into /proc/sys/vm. These files allow user apps to understand writeback behaviour over time and learn how it is impacting their performance. # cat /proc/sys/vm/pages_dirtied 3747 # cat /proc/sys/vm/pages_entered_writeback 3618 These two new files are necessary to give visibility into writeback behaviour. We have /proc/diskstats which lets us understand the io in the block layer. We have blktrace for more in depth understanding. We have e2fsprogs and debugsfs to give insight into the file systems behaviour, but we don't offer our users the ability understand what writeback is doing. There is no way to know how active it is over the whole system, if it's falling behind or to quantify it's efforts. With these values exported users can easily see how much data applications are sending through writeback and also at what rates writeback is processing this data. Comparing the rates of change between the two allow developers to see when writeback is not able to keep up with incoming traffic and the rate of dirty memory being sent to the IO back end. This allows folks to understand their io workloads and track kernel issues. Non kernel engineers at Google often use these counters to solve puzzling performance problems. There is a strong case for putting these files in /proc/sys/vm. It was Christoph's suggestion that made me realize that /proc/sys/vm is their proper home. Most if not all the tunables for writeback are there. When one is trying to find the state of the system's writeback activity that's the directory. In addition while the internals of writeback will probably change the use of the dirty and writeback states and pages will be around for a long time. Exposing them in /proc/sys/vm should be acceptable. Having these pages_dirtied and pages_entered_writeback in /proc/vmstat to me feels like a way to make sure that users who would need them won't find them unless they are reading source. And these are folks who aren't reading source. Patch #3 adds dirty thresholds to /proc/vmstat. # grep threshold /proc/vmstat nr_pages_dirty_threshold 409111 nr_pages_dirty_background_threshold 818223 The files that report the dirty thresholds belong in /proc/vmstat. They are meant for application writers so should not be in debugfs. But since they are more related to internals of writeback, albeit internals that are fundamental to how it works, /proc/sys/vm is not appropriate. Michael Rubin (3): mm: helper functions for dirty and writeback accounting writeback: Adding pages_dirtied and pages_entered_writeback writeback: Reporting dirty thresholds in /proc/vmstat Documentation/sysctl/vm.txt | 20 +++++++++++++++--- drivers/base/node.c | 14 +++++++++++++ fs/ceph/addr.c | 8 +----- fs/nilfs2/segment.c | 2 +- include/linux/mm.h | 1 + include/linux/mmzone.h | 4 +++ include/linux/writeback.h | 9 ++++++++ kernel/sysctl.c | 14 +++++++++++++ mm/page-writeback.c | 45 ++++++++++++++++++++++++++++++++++++++++-- mm/vmstat.c | 10 +++++++++ 10 files changed, 113 insertions(+), 14 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
