Memory Management

From eLinux.org
Revision as of 20:34, 21 September 2009 by Tim Bird (Talk | contribs) (Enhanced Out-Of-Memory (OOM) handling: update thread link)

Jump to: navigation, search

This page has information about various memory management projects and activities which are of interest to embedded Linux developers.

Areas of Interest

Most of these areas have wider reaching implications, but are relatively simpler in the embedded case, largely thanks to not having to contend with swap and things of that nature. Simpler memory management as well as vendors not afraid of deviation from mainline for product programs makes for an excellent playground for experimenting with new things in the memory management and virtual memory space.

Memory Measurement

Analyzing the amount of system memory in use and available is trickier than it sounds.

Huge/large/superpages

  • This applies to both transparent large page usage as well as the more static usage models, primarily relating to work outside of the hugetlb interface/libhugetlbfs.
  • Embedded systems suffer from very small TLBs generally using PAGE_SIZE'd pages (4kB) for coverage. In most cases this places the system under very heavy pressure for any kind of userspace work, and very visibly degrading performance, with most applications taking anywhere from 5-40% of their time on the CPU servicing page faults.
  • Preliminary discussion on this subject as well as links to additional information is happening through the wiki here: Huge Pages

Page cache compression

  • This relates to using various compression algorithms for performing run-time compression and decompression of page cache pages, specifically aimed at both reducing memory pressure as well as helping performance in certain workloads.
  • More information can be found on the wiki here CompressedCaching as well as at the SF Compressed Caching home page.

Reserving (and accessing) the top of memory on startup

A quote from Todd's email on how to use the reserved physical memory in "mem=".


Given that you have a fixed address for your memory, and is already reserved, the easier way to use it is by calling mmap() over the /dev/ mem device, use 0 as the start address, and the physical address of the reserved memory as the offset. The flags could be MAP_WRITE| MAP_READ. That will return you a pointer on user space for your memory mapped by the kernel. For example

If your SDRAM base address is 0x80000000 and your memory is of 64MB, but you use the cmdline mem=60M to reserve 4MB at the end. Then your reserved memory will be at 0x83c00000, so all you need to do is

int fd;
char *reserved_memory;

fd = open("/dev/mem",O_RDWR);
reserved_memory = (char *) mmap(0,4*1024*1024,PROT_READ| PROT_WRITE,MAP_SHARED,fd,0x83c00000);

Enhanced Out-Of-Memory (OOM) handling

Several technologies have been developed and suggested for improving the handling out-of-memory conditions with Linux systems.

See http://linux-mm.org/OOM_Killer for information about the OOM killer in the Linux kernel.

Part of OOM avoidance is for the kernel to have an accurate measure of memory utilization. See Accurate Memory Measurement for information on technology in this area.

Here are some technologies that I know about (these need to be researched and documented better):

  • Memory usage limit notification (By Embedded Alley, sponsored by CE Linux Forum)
    • This patch updates the Memory Controller cgroup to add a configurable memory usage limit notification. The feature was presented at the April 2009 Embedded Linux Conference.
    • See http://lwn.net/Articles/328403/ - article about first submission to LKML in April 2009
    • http://lkml.org/lkml/2009/7/7/410 - thread about second submission to LKML in July 2009
  • mem_notify patches
    • This set of patches provided a mechanism to notify user-space when memory is getting low, allowing for application-based handling of the condition. These patches were submitted in January 2008.
    • This patch cannot be applied to versions beyond 2.6.28 because the memory management reclaiming sequence have changed.
    • See http://lwn.net/Articles/267013/
  • Google per-cgroup OOM handler
  • Nokia OOM enhancements
User "oak" writes (commenting on the mem_notify patches):

Posted Feb 3, 2008 14:02 UTC (Sun) by oak (guest, #2786) [Link]

...

I thought the point of the patch is for user-space to be able to do the  
memory management in *manageable places* in code.   As mentioned earlier, 
a lot of user-space code[1] doesn't handle memory allocation failures. And 
even if it's supposed to be, it can be hard to verify (test) that the 
failures are handled in *all* cases properly.  If user-space can get a 
pre-notification of a low-memory situation, it can in suitable place in 
code free memory so that further allocations will succeed (with higher 
propability). 

That also allows doing somehing like what maemo does.  If system gets 
notified about kernel low memory shortage, it kills processes which have 
notified it that they are in "background-killable" state (saved their UI 
state, able to restore it and not currently visible to user). I think it 
also notifies applications (currently) through D-BUS about low memory 
condition. Applications visible to user or otherwise non-background 
killable are then supposed to free their caches and/or disable features 
that could take a lot of additional memory.  If the caches are from heap 
instead of memory mapped, it's less likely to help because of heap 
fragmentation and it requiring more work/time though.

Type-based memory allocation (old)

This is a mechanism (prototyped in the 2.4 kernel by Sony and Panasonic) to allow the kernel to allocate different types of memory for different sections of a program, based on user policy.

See Memory Type Based Allocation

Additional Resources/Mailing Lists

  • LinuxMM - links to various sub-projects, and acts as a centralized point for discussion relating to memory management topics (linux-mm mailing list and archives).