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Diffstat (limited to 'Documentation/admin-guide/mm/ksm.rst')
| -rw-r--r-- | Documentation/admin-guide/mm/ksm.rst | 144 |
1 files changed, 139 insertions, 5 deletions
diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst index 874eb0c77d34..a639cac12477 100644 --- a/Documentation/admin-guide/mm/ksm.rst +++ b/Documentation/admin-guide/mm/ksm.rst @@ -1,5 +1,3 @@ -.. _admin_guide_ksm: - ======================= Kernel Samepage Merging ======================= @@ -9,7 +7,7 @@ Overview KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation, -and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/ +and http://lwn.net/Articles/306704/ and https://lwn.net/Articles/330589/ KSM was originally developed for use with KVM (where it was known as Kernel Shared Memory), to fit more virtual machines into physical memory, @@ -22,7 +20,7 @@ content which can be replaced by a single write-protected page (which is automatically copied if a process later wants to update its content). The amount of pages that KSM daemon scans in a single pass and the time between the passes are configured using :ref:`sysfs -intraface <ksm_sysfs>` +interface <ksm_sysfs>` KSM only merges anonymous (private) pages, never pagecache (file) pages. KSM's merged pages were originally locked into kernel memory, but can now @@ -52,7 +50,7 @@ with EAGAIN, but more probably arousing the Out-Of-Memory killer. If KSM is not configured into the running kernel, madvise MADV_MERGEABLE and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was built with CONFIG_KSM=y, those calls will normally succeed: even if the -the KSM daemon is not currently running, MADV_MERGEABLE still registers +KSM daemon is not currently running, MADV_MERGEABLE still registers the range for whenever the KSM daemon is started; even if the range cannot contain any pages which KSM could actually merge; even if MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE. @@ -82,6 +80,9 @@ pages_to_scan how many pages to scan before ksmd goes to sleep e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``. + The pages_to_scan value cannot be changed if ``advisor_mode`` has + been set to scan-time. + Default: 100 (chosen for demonstration purposes) sleep_millisecs @@ -157,8 +158,44 @@ stable_node_chains_prune_millisecs scan. It's a noop if not a single KSM page hit the ``max_page_sharing`` yet. +smart_scan + Historically KSM checked every candidate page for each scan. It did + not take into account historic information. When smart scan is + enabled, pages that have previously not been de-duplicated get + skipped. How often these pages are skipped depends on how often + de-duplication has already been tried and failed. By default this + optimization is enabled. The ``pages_skipped`` metric shows how + effective the setting is. + +advisor_mode + The ``advisor_mode`` selects the current advisor. Two modes are + supported: none and scan-time. The default is none. By setting + ``advisor_mode`` to scan-time, the scan time advisor is enabled. + The section about ``advisor`` explains in detail how the scan time + advisor works. + +adivsor_max_cpu + specifies the upper limit of the cpu percent usage of the ksmd + background thread. The default is 70. + +advisor_target_scan_time + specifies the target scan time in seconds to scan all the candidate + pages. The default value is 200 seconds. + +advisor_min_pages_to_scan + specifies the lower limit of the ``pages_to_scan`` parameter of the + scan time advisor. The default is 500. + +adivsor_max_pages_to_scan + specifies the upper limit of the ``pages_to_scan`` parameter of the + scan time advisor. The default is 30000. + The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``: +general_profit + how effective is KSM. The calculation is explained below. +pages_scanned + how many pages are being scanned for ksm pages_shared how many shared pages are being used pages_sharing @@ -167,12 +204,21 @@ pages_unshared how many pages unique but repeatedly checked for merging pages_volatile how many pages changing too fast to be placed in a tree +pages_skipped + how many pages did the "smart" page scanning algorithm skip full_scans how many times all mergeable areas have been scanned stable_node_chains the number of KSM pages that hit the ``max_page_sharing`` limit stable_node_dups number of duplicated KSM pages +ksm_zero_pages + how many zero pages that are still mapped into processes were mapped by + KSM when deduplicating. + +When ``use_zero_pages`` is/was enabled, the sum of ``pages_sharing`` + +``ksm_zero_pages`` represents the actual number of pages saved by KSM. +if ``use_zero_pages`` has never been enabled, ``ksm_zero_pages`` is 0. A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing`` @@ -184,6 +230,94 @@ The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the ``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must be increased accordingly. +Monitoring KSM profit +===================== + +KSM can save memory by merging identical pages, but also can consume +additional memory, because it needs to generate a number of rmap_items to +save each scanned page's brief rmap information. Some of these pages may +be merged, but some may not be abled to be merged after being checked +several times, which are unprofitable memory consumed. + +1) How to determine whether KSM save memory or consume memory in system-wide + range? Here is a simple approximate calculation for reference:: + + general_profit =~ ksm_saved_pages * sizeof(page) - (all_rmap_items) * + sizeof(rmap_item); + + where ksm_saved_pages equals to the sum of ``pages_sharing`` + + ``ksm_zero_pages`` of the system, and all_rmap_items can be easily + obtained by summing ``pages_sharing``, ``pages_shared``, ``pages_unshared`` + and ``pages_volatile``. + +2) The KSM profit inner a single process can be similarly obtained by the + following approximate calculation:: + + process_profit =~ ksm_saved_pages * sizeof(page) - + ksm_rmap_items * sizeof(rmap_item). + + where ksm_saved_pages equals to the sum of ``ksm_merging_pages`` and + ``ksm_zero_pages``, both of which are shown under the directory + ``/proc/<pid>/ksm_stat``, and ksm_rmap_items is also shown in + ``/proc/<pid>/ksm_stat``. The process profit is also shown in + ``/proc/<pid>/ksm_stat`` as ksm_process_profit. + +From the perspective of application, a high ratio of ``ksm_rmap_items`` to +``ksm_merging_pages`` means a bad madvise-applied policy, so developers or +administrators have to rethink how to change madvise policy. Giving an example +for reference, a page's size is usually 4K, and the rmap_item's size is +separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture. +so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU +or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped, +because the ksm profit is approximately zero or negative. + +Monitoring KSM events +===================== + +There are some counters in /proc/vmstat that may be used to monitor KSM events. +KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW +or on swapping in copy. Those events could help users evaluate whether or how +to use KSM. For example, if cow_ksm increases too fast, user may decrease the +range of madvise(, , MADV_MERGEABLE). + +cow_ksm + is incremented every time a KSM page triggers copy on write (COW) + when users try to write to a KSM page, we have to make a copy. + +ksm_swpin_copy + is incremented every time a KSM page is copied when swapping in + note that KSM page might be copied when swapping in because do_swap_page() + cannot do all the locking needed to reconstitute a cross-anon_vma KSM page. + +Advisor +======= + +The number of candidate pages for KSM is dynamic. It can be often observed +that during the startup of an application more candidate pages need to be +processed. Without an advisor the ``pages_to_scan`` parameter needs to be +sized for the maximum number of candidate pages. The scan time advisor can +changes the ``pages_to_scan`` parameter based on demand. + +The advisor can be enabled, so KSM can automatically adapt to changes in the +number of candidate pages to scan. Two advisors are implemented: none and +scan-time. With none, no advisor is enabled. The default is none. + +The scan time advisor changes the ``pages_to_scan`` parameter based on the +observed scan times. The possible values for the ``pages_to_scan`` parameter is +limited by the ``advisor_max_cpu`` parameter. In addition there is also the +``advisor_target_scan_time`` parameter. This parameter sets the target time to +scan all the KSM candidate pages. The parameter ``advisor_target_scan_time`` +decides how aggressive the scan time advisor scans candidate pages. Lower +values make the scan time advisor to scan more aggresively. This is the most +important parameter for the configuration of the scan time advisor. + +The initial value and the maximum value can be changed with +``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The default +values are sufficient for most workloads and use cases. + +The ``pages_to_scan`` parameter is re-calculated after a scan has been completed. + + -- Izik Eidus, Hugh Dickins, 17 Nov 2009 |