Google Summer of Code 2019

This page lists accepted projects that were completed during Google Summer of Code 2019. For the list of ideas go here.

Student final submissions

Abhishek Dubey - Optimize the pre-dump algorithm

Final report: https://dubeyabhishek.github.io/GSoC19CRIU/

Summary: Optimize the pre-dump algorithm to avoid pinning to many memory in RAM

Current pre-dump mode is used to write task memory contents into image files w/o stopping the task for too long. It does this by stopping the task, infecting it and draining all the memory into a set of pipes. Then the task is cured, resumed and the pipes' contents is written into images (maybe a page server). Unfortunately, this approach creates a big stress on the memory subsystem, as keeping all memory in pipes creates a lot of unreclaimable memory (pages in pipes are not swappable), as well as the number of pipes themselves can be huge, as one pipe doesn't store more than a fixed amount of data (see pipe(7) man page).

A solution for this problem is to use a sys_read_process_vm() syscall, which will mitigate all of the above. To do this we need to allocate a temporary buffer in criu, then walk the target process vm by copying the memory piece-by-piece into it, then flush the data into image (or page server), and repeat.

Ideally there should be sys_splice_process_vm() syscall in the kernel, that does the same as the read_process_vm does, but vmsplices the data

Links:

• Memory pre dump
• https://github.com/checkpoint-restore/criu/issues/351
• Memory dumping and restoring, Memory changes tracking
• process_vm_readv(2) vmsplice(2) RFC for splice_process_vm syscall

Details:

• Skill level: advanced
• Language: C
• Mentor: Pavel Emelianov <xemul@virtuozzo.com>
• Suggested by: Pavel Emelianov <xemul@virtuozzo.com>