Performance research

Written here are performance issues found.

Timing stats of live migration of a small container with 11 tasks is

Total time ~3.5 seconds
Frozen time ~3.0 seconds
- Pre-dump stages ~0.5 seconds each
- Restore time ~1.9 seconds
- Images transfer time ~0.3 seconds

Below is the list of issues found

Dump

Surprisingly, but the mem-drain time is not the biggest. It's "only" ~0.02 seconds. There are places in code that take longer.

`parse_smaps`

Time spent in this routine is up to 0.2 seconds on dump. This one exploits /proc heavily. For a container with 11 tasks the syscall stats look like

   834 read
  1451 fstat
  1462 close
  1642 openat

while opens and stats happen on

   193 openat(4, "map-symlink", O_RDONLY) = -1 ENOENT (No such file or directory)
  1438 openat(4, "map-symlink", O_RDONLY) = 5
    11 openat(AT_FDCWD, "/proc/$pid/map_files", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 4

Restore

Fork vs VMA restore

We restore task's mappings before it goes forking to handle COW. This effectively serializes forking.

Restoring VMAs

There are 4 stages in VMA restore. Relative times of each are below

Reading images 1%
Mapping huuge premap area << 1%
(Re-)mapping sub-areas 73%
Filling area with data 26%

The 3rd stage has two parts. With timings:

Opening filemap fd 85%
Maping vma 15%

Opening files for mappings

The get_filemap_fd() opens new fd every time. If a file is mapped several times (e.g. -- a library) we can share one fd for that.

Staging

When restoring a single task CRIU uses stages of restoring which slows things down. Need either special-care the single task restore, or introduce fine-grained locking for such things.