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    Subject[PATCH 20/33] memory controller add documentation
    From: Balbir Singh <>
    (container->cgroup renaming by Paul Menage <>)

    Signed-off-by: Balbir Singh <>
    Signed-off-by: Paul Menage <>

    Documentation/controllers/memory.txt | 259 +++++++++++++++++++++++++
    1 files changed, 259 insertions(+)
    diff -puN /dev/null Documentation/controllers/memory.txt
    --- /dev/null
    +++ a/Documentation/controllers/memory.txt
    @@ -0,0 +1,259 @@
    +Memory Controller
    +Salient features
    +a. Enable control of both RSS (mapped) and Page Cache (unmapped) pages
    +b. The infrastructure allows easy addition of other types of memory to control
    +c. Provides *zero overhead* for non memory controller users
    +d. Provides a double LRU: global memory pressure causes reclaim from the
    + global LRU; a cgroup on hitting a limit, reclaims from the per
    + cgroup LRU
    +NOTE: Page Cache (unmapped) also includes Swap Cache pages as a subset
    +and will not be referred to explicitly in the rest of the documentation.
    +Benefits and Purpose of the memory controller
    +The memory controller isolates the memory behaviour of a group of tasks
    +from the rest of the system. The article on LWN [12] mentions some probable
    +uses of the memory controller. The memory controller can be used to
    +a. Isolate an application or a group of applications
    + Memory hungry applications can be isolated and limited to a smaller
    + amount of memory.
    +b. Create a cgroup with limited amount of memory, this can be used
    + as a good alternative to booting with mem=XXXX.
    +c. Virtualization solutions can control the amount of memory they want
    + to assign to a virtual machine instance.
    +d. A CD/DVD burner could control the amount of memory used by the
    + rest of the system to ensure that burning does not fail due to lack
    + of available memory.
    +e. There are several other use cases, find one or use the controller just
    + for fun (to learn and hack on the VM subsystem).
    +1. History
    +The memory controller has a long history. A request for comments for the memory
    +controller was posted by Balbir Singh [1]. At the time the RFC was posted
    +there were several implementations for memory control. The goal of the
    +RFC was to build consensus and agreement for the minimal features required
    +for memory control. The first RSS controller was posted by Balbir Singh[2]
    +in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
    +RSS controller. At OLS, at the resource management BoF, everyone suggested
    +that we handle both page cache and RSS together. Another request was raised
    +to allow user space handling of OOM. The current memory controller is
    +at version 6; it combines both mapped (RSS) and unmapped Page
    +Cache Control [11].
    +2. Memory Control
    +Memory is a unique resource in the sense that it is present in a limited
    +amount. If a task requires a lot of CPU processing, the task can spread
    +its processing over a period of hours, days, months or years, but with
    +memory, the same physical memory needs to be reused to accomplish the task.
    +The memory controller implementation has been divided into phases. These
    +1. Memory controller
    +2. mlock(2) controller
    +3. Kernel user memory accounting and slab control
    +4. user mappings length controller
    +The memory controller is the first controller developed.
    +2.1. Design
    +The core of the design is a counter called the res_counter. The res_counter
    +tracks the current memory usage and limit of the group of processes associated
    +with the controller. Each cgroup has a memory controller specific data
    +structure (mem_cgroup) associated with it.
    +2.2. Accounting
    + +--------------------+
    + | mem_cgroup |
    + | (res_counter) |
    + +--------------------+
    + / ^ \
    + / | \
    + +---------------+ | +---------------+
    + | mm_struct | |.... | mm_struct |
    + | | | | |
    + +---------------+ | +---------------+
    + |
    + + --------------+
    + |
    + +---------------+ +------+--------+
    + | page +----------> page_cgroup|
    + | | | |
    + +---------------+ +---------------+
    + (Figure 1: Hierarchy of Accounting)
    +Figure 1 shows the important aspects of the controller
    +1. Accounting happens per cgroup
    +2. Each mm_struct knows about which cgroup it belongs to
    +3. Each page has a pointer to the page_cgroup, which in turn knows the
    + cgroup it belongs to
    +The accounting is done as follows: mem_cgroup_charge() is invoked to setup
    +the necessary data structures and check if the cgroup that is being charged
    +is over its limit. If it is then reclaim is invoked on the cgroup.
    +More details can be found in the reclaim section of this document.
    +If everything goes well, a page meta-data-structure called page_cgroup is
    +allocated and associated with the page. This routine also adds the page to
    +the per cgroup LRU.
    +2.2.1 Accounting details
    +All mapped pages (RSS) and unmapped user pages (Page Cache) are accounted.
    +RSS pages are accounted at the time of page_add_*_rmap() unless they've already
    +been accounted for earlier. A file page will be accounted for as Page Cache;
    +it's mapped into the page tables of a process, duplicate accounting is carefully
    +avoided. Page Cache pages are accounted at the time of add_to_page_cache().
    +The corresponding routines that remove a page from the page tables or removes
    +a page from Page Cache is used to decrement the accounting counters of the
    +2.3 Shared Page Accounting
    +Shared pages are accounted on the basis of the first touch approach. The
    +cgroup that first touches a page is accounted for the page. The principle
    +behind this approach is that a cgroup that aggressively uses a shared
    +page will eventually get charged for it (once it is uncharged from
    +the cgroup that brought it in -- this will happen on memory pressure).
    +2.4 Reclaim
    +Each cgroup maintains a per cgroup LRU that consists of an active
    +and inactive list. When a cgroup goes over its limit, we first try
    +to reclaim memory from the cgroup so as to make space for the new
    +pages that the cgroup has touched. If the reclaim is unsuccessful,
    +an OOM routine is invoked to select and kill the bulkiest task in the
    +The reclaim algorithm has not been modified for cgroups, except that
    +pages that are selected for reclaiming come from the per cgroup LRU
    +2. Locking
    +The memory controller uses the following hierarchy
    +1. zone->lru_lock is used for selecting pages to be isolated
    +2. mem->lru_lock protects the per cgroup LRU
    +3. lock_page_cgroup() is used to protect page->page_cgroup
    +3. User Interface
    +0. Configuration
    +a. Enable CONFIG_CGROUPS
    +1. Prepare the cgroups
    +# mkdir -p /cgroups
    +# mount -t cgroup none /cgroups -o memory
    +2. Make the new group and move bash into it
    +# mkdir /cgroups/0
    +# echo $$ > /cgroups/0/tasks
    +Since now we're in the 0 cgroup,
    +We can alter the memory limit:
    +# echo -n 6000 > /cgroups/0/memory.limit
    +We can check the usage:
    +# cat /cgroups/0/memory.usage
    +The memory.failcnt field gives the number of times that the cgroup limit was
    +4. Testing
    +Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].
    +Apart from that v6 has been tested with several applications and regular
    +daily use. The controller has also been tested on the PPC64, x86_64 and
    +UML platforms.
    +4.1 Troubleshooting
    +Sometimes a user might find that the application under a cgroup is
    +terminated. There are several causes for this:
    +1. The cgroup limit is too low (just too low to do anything useful)
    +2. The user is using anonymous memory and swap is turned off or too low
    +A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
    +some of the pages cached in the cgroup (page cache pages).
    +4.2 Task migration
    +When a task migrates from one cgroup to another, it's charge is not
    +carried forward. The pages allocated from the original cgroup still
    +remain charged to it, the charge is dropped when the page is freed or
    +4.3 Removing a cgroup
    +A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
    +cgroup might have some charge associated with it, even though all
    +tasks have migrated away from it. If some pages are still left, after following
    +the steps listed in sections 4.1 and 4.2, check the Swap Cache usage in
    +/proc/meminfo to see if the Swap Cache usage is showing up in the
    +cgroups memory.usage counter. A simple test of swapoff -a and swapon -a
    +should free any pending Swap Cache usage.
    +4.4 Choosing what to account -- Page Cache (unmapped) vs RSS (mapped)?
    +The type of memory accounted by the cgroup can be limited to just
    +mapped pages by writing "1" to memory.control_type field
    +echo -n 1 > memory.control_type
    +5. TODO
    +1. Add support for accounting huge pages (as a separate controller)
    +2. Improve the user interface to accept/display memory limits in KB or MB
    + rather than pages (since page sizes can differ across platforms/machines).
    +3. Make cgroup lists per-zone
    +4. Make per-cgroup scanner reclaim not-shared pages first
    +5. Teach controller to account for shared-pages
    +6. Start reclamation when the limit is lowered
    +7. Start reclamation in the background when the limit is
    + not yet hit but the usage is getting closer
    +8. Create per zone LRU lists per cgroup
    +Overall, the memory controller has been a stable controller and has been
    +commented and discussed quite extensively in the community.
    +1. Singh, Balbir. RFC: Memory Controller,
    +2. Singh, Balbir. Memory Controller (RSS Control),
    +3. Emelianov, Pavel. Resource controllers based on process cgroups
    +4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
    +5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
    +6. Menage, Paul. Control Groups v10,
    +7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
    + subsystem (v3),
    +8. Singh, Balbir. RSS controller V2 test results (lmbench),
    +9. Singh, Balbir. RSS controller V2 AIM9 results
    +10. Singh, Balbir. Memory controller v6 results,
    +11. Singh, Balbir. Memory controller v6,
    +12. Corbet, Jonathan, Controlling memory use in cgroups,
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