Page server

Page server is a component of CRIU that allows to copy (rather than dump) user memory to a destination system during the course of live migration. It is also used for lazy migration.

Rationale

For process tree migration, the biggest part of transfer data is the memory used by the processes. Therefore, optimizing this memory transfer would be beneficial.

Memory migration without page server

Without the page server, migrating the user memory pages consists of:

dumping (writing) the memory to files on disk;
reading the files and sending the data over the network to the destination host;
receiving files on the destination host, writing to files on disk;
restoring by reading the files into memory.

In other words, all the memory is written to the disk twice, and read from the disk also twice. It incurs significant I/O overhead and slows down the migration. The overhead is further multiplied when using criu pre-dump (such as for iterative migration), as in this case memory is dumped not once but a few times.

A way to mitigate the disk I/O overhead is to use tmpfs, a filesystem that use RAM as a storage (see Disk-less migration for details). This eliminates the disk I/O, but not the double read/write. To eliminate it, a page server mechanism was implemented.

Operation

Memory migration with page server

When using page server, the memory is dumped not to disk, but directly to network, thus eliminating any disk reads/writes on the source. On the destination system, a page server runs, receiving the data from network and writing it to files on disk or tmpfs.

Pages deduplication

When iterative memory dumping feature (criu pre-dump) is used, memory is sent to page server a few times. The page server can automatically deduplicate pages, by punching holes in parent images where the child image is replacing an existing page. This functionality is turned on by --auto-dedup option of criu page-server command.

Usage

You would want to use page server feature when performing a live migration. In most cases, just use P.Haul which handles the details for you. Otherwise, read on.

Running page server

First, run a page server on a destination node:

[dst]# criu page-server --images-dir $DIR --port $PORT [--auto-dedup]

The options are:

--images-dir $DIR: A directory to write memory images to. To speed things up, you might want to use tmpfs (see disk-less migration).

--port $PORT: A port number to listen at.

--auto-dedup: Perform auto deduplication of images. Useful with iterative memory dumps.

criu pre-dump/dump

With page-server, you need to specify additional arguments to criu pre-dump or criu dump commands. They are:

--page-server: Send pages to a page server (rather than writing to disk files)

--address $IPADDR: IP address of the page server (destination host).

--port $PORT: Port number the page servers is listening at.

Video

To view an example video of using a page server, please go to https://asciinema.org/a/15847.

External links

Using page server: https://asciinema.org/a/15847

Page server

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