Re: [RESEND] shm: shm exit scalability fixes

[Anton Blanchard <anton@samba.org>]

Hi David,

> > Anton wrote a simple test to cause the issue:
> > 
> > http://ozlabs.org/~anton/junkcode/bust_shm_exit.c
> 
> I'm actually in the process of adding shm microbenchmarks to
> perf-bench so I might steal this :-)

Sounds good!

> Are you seeing this issue in any real world setups? While the program
> does stress the path you mention quite well, I fear it is very
> unrealistic... how many shared mem segments do real applications
> actually use/create for scaling issues to appear?

As Jack mentioned, we were asked to debug a box that was crawling. Each
process took over 10 minutes to execute which made it very hard to
analyse. We eventually narrowed it down to this.

> I normally wouldn't mind optimizing synthetic cases like this, but a
> quick look at patch 1/3 shows that we're adding an extra overhead (16
> bytes) in the task_struct.

The testcase is synthetic but I wrote it based on the application that
would, given enough time, take the box down.

> We have the shmmni limit (and friends) for that.

If we want to use this to guard against the problem, we may need to
drop shmmni. Looking at my notes, I could take down a box with 4096
segments and 16 threads. This is where I got to before it disappeared:

# ./bust_shm_exit 4096 16
# uptime
03:00:50 up 8 days, 18:05  5 users,load average: 6076.98, 2494.09, 910.37

Anton