Memory controller: add document
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com> Cc: Pavel Emelianov <xemul@openvz.org> Cc: Paul Menage <menage@google.com> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Cc: Kirill Korotaev <dev@sw.ru> Cc: Herbert Poetzl <herbert@13thfloor.at> Cc: David Rientjes <rientjes@google.com> Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Balbir Singh
Linus Torvalds
parent
e9685a03c8
commit
1b6df3aa45
1 changed files with 259 additions and 0 deletions
259
Documentation/controllers/memory.txt
Normal file
259
Documentation/controllers/memory.txt
Normal file
|
|
@ -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
|
||||
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 6000 > /cgroups/0/memory.limit
|
||||
|
||||
We can check the usage:
|
||||
# cat /cgroups/0/memory.usage
|
||||
25
|
||||
|
||||
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 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/
|
||||
Loading…
Reference in a new issue