Re: [RESEND] shm: shm exit scalability fixes

[Michael Ellerman <mpe@ellerman.id.au>]

On Wed, 2014-06-18 at 12:53 +1000, Anton Blanchard wrote:
> > 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.
> 
> > 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

I win, using 4096 segments and 16 threads:

 # uptime
 13:50:46 up 1 day, 19:41,  2 users,  load average: 7621.57, 1718.39, 943.13
 13:52:35 up 1 day, 19:43,  2 users,  load average: 15422.64, 7409.90, 3156.82

That's on a 16 cpu box running 3.16-rc1.

In contrast, if you run it with 1 segment and 16 threads it maxes out about:

 # uptime
 13:58:00 up 1 min,  2 users,  load average: 1.81, 0.46, 0.15

And the box is entirely responsive.

cheers