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
are:

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
cgroup.

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
cgroup.

The reclaim algorithm has not been modified for cgroups, except that
pages that are selected for reclaiming come from the per cgroup LRU
list.

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
b. Enable CONFIG_RESOURCE_COUNTERS
c. Enable CONFIG_CGROUP_MEM_CONT

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 4M > /cgroups/0/memory.limit_in_bytes

NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
mega or gigabytes.

# cat /cgroups/0/memory.limit_in_bytes
4194304 Bytes

NOTE: The interface has now changed to display the usage in bytes
instead of pages

We can check the usage:
# cat /cgroups/0/memory.usage_in_bytes
1216512 Bytes

A successful write to this file does not guarantee a successful set of
this limit to the value written into the file.  This can be due to a
number of factors, such as rounding up to page boundaries or the total
availability of memory on the system.  The user is required to re-read
this file after a write to guarantee the value committed by the kernel.

# echo -n 1 > memory.limit_in_bytes
# cat memory.limit_in_bytes
4096 Bytes

The memory.failcnt field gives the number of times that the cgroup limit was
exceeded.

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
reclaimed.

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_in_bytes 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

Summary

Overall, the memory controller has been a stable controller and has been
commented and discussed quite extensively in the community.

References

1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
2. Singh, Balbir. Memory Controller (RSS Control),
   http://lwn.net/Articles/222762/
3. Emelianov, Pavel. Resource controllers based on process cgroups
   http://lkml.org/lkml/2007/3/6/198
4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
   http://lkml.org/lkml/2007/4/9/74
5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
   http://lkml.org/lkml/2007/5/30/244
6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
   subsystem (v3), http://lwn.net/Articles/235534/
8. Singh, Balbir. RSS controller V2 test results (lmbench),
   http://lkml.org/lkml/2007/5/17/232
9. Singh, Balbir. RSS controller V2 AIM9 results
   http://lkml.org/lkml/2007/5/18/1
10. Singh, Balbir. Memory controller v6 results,
    http://lkml.org/lkml/2007/8/19/36
11. Singh, Balbir. Memory controller v6, http://lkml.org/lkml/2007/8/17/69
12. Corbet, Jonathan, Controlling memory use in cgroups,
    http://lwn.net/Articles/243795/