[PATCH 0/2] Adding four writeback files in /proc/sys/vm
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From: Michael Rubin on 3 Aug 2010 18:30 Patch #1 sets up some helper functions for accounting. Patch #2 adds some writeback files for visibility To help developers and applications gain visibility into writeback behaviour adding four 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 # cat /proc/sys/vm/dirty_threshold_kbytes 816673 # cat /proc/sys/vm/dirty_background_threshold_kbytes 408336 The files fall into two groups. pages_dirtied and pages_entered_writeback: 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 indepth 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 non-debugfs way to know how active it is, if it's falling behind or to quantify it's efforts on a system. 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. dirty_threshold_kbytes and dirty_background_threshold kbytes: We already expose these thresholds in /proc/sys/vm with dirty_background_ratio and background_ratio. What's frustrating about the ratio variables and the need for these are that they are not honored by the kernel. Instead the kernel may alter the number requested without giving the user any indication that is the case. An app developer can set the ratio to 2% but end up with 5% as get_dirty_limits makes sure it is never lower than 5% when set from the ratio. Arguably that can be fixed too but the limits which decide whether writeback is invoked to aggressively clean dirty pages is dependent on changing page state retrieved in determine_dirtyable_memory. It makes understanding when the kernel decides to writeback data a moving target that no app can ever determine. With these thresholds visible and collected over time it gives apps a chance to know why writeback happened, or why it did not. As systems get larger and larger RAM developers use the ratios to predict when their workloads will see writeback invoked. Today there is no way to accurately indicate what the kernel will use to kick off writeback. Hence the need for these two new files. Michael Rubin (2): mm: helper functions for dirty and writeback accounting writeback: Adding four read-only files to /proc/sys/vm Documentation/sysctl/vm.txt | 41 ++++++++++++++++++++++-- drivers/base/node.c | 14 ++++++++ fs/ceph/addr.c | 8 +--- fs/nilfs2/segment.c | 2 +- include/linux/mm.h | 1 + include/linux/mmzone.h | 2 + include/linux/writeback.h | 17 ++++++++++ kernel/sysctl.c | 28 ++++++++++++++++ mm/page-writeback.c | 73 +++++++++++++++++++++++++++++++++++++++++-- mm/vmstat.c | 2 + 10 files changed, 174 insertions(+), 14 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |