Memory changes tracking

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CRIU can detect what memory pages a task (or tasks) has changed since some moment of time. This page describes why this is required, how it works and how to use it.

Why do we need to track memory changed

There are several scenarios where detecting what parts of memory has changed is required:

Incremental dumps
When you take a series of dumps from a process tree, it is a very good optimization not to dump all the memory every time, but get only those memory pages that has changed since previous dump
Smaller freeze time for big applications
When a task uses a LOT of memory, dumping it may take time and during all this time this task should be frozen. To reduce the freeze time we can
  • get memory from task and start writing it in images
  • freeze task and get only changed memory from it
Live migration
When doing live migration, a lot of time is used by the procedure of copying tasks' memory to the destination host. Note that the processes are frozen during that time. Acting like in the previous example also reduces the freeze time, i.e. the live migration becomes more live.

How we track memory changes

In order to find out which memory pages have changed, we patched the kernel. Tracking the memory changes consists of two steps:

  • ask the kernel to keep track of memory changes (by writing 4 into /proc/$pid/clear_refs file for each $pid we are interested in).

and, after a while,

  • get the list of modified pages of a process by reading its /proc/$pid/pagemap file and looking at so called soft-dirty bit in the pagemap entries.

During the first step, kernel will re-map all the tasks' mapping in read-only manner. If a task then tries to write into any of its pages, a page fault will occur, and the kernel will note which page is being written to. Reading the pagemap file reveals this information.

How to use this with CRIU

First of all, the

# criu check --feature mem_dirty_track

command should say the feature is supported. The memory changes tracking was initially merged into Linux kernel v3.11, and was further polished until v3.18 (see for details).

There are several command line options to use the functionality:

--prev-images-dir option
This option is used to provide the path where images from a previous dump or pre-dump (see below) action reside. If possible, CRIU will dump only the memory pages that have been modified since that time.
--track-mem option
This option makes CRIU to reset memory changes tracker. If done, the next dump --prev-images-dir will have chances to successfully find not changed pages.
pre-dump action
This action dumps only part of the information about processes and does that by keeping tasks frozen for the shortest possible time. The images generated by pre-dump cannot and should not be used for restore. After this action the proper dump should be performed with properly configured --prev-images-dir path.

See also

External links