Re: [RESEND] shm: shm exit scalability fixes

[Davidlohr Bueso]

On Tue, 2014-06-17 at 12:27 -0500, Jack Miller wrote:
> [ RESEND note: Adding relevant CCs, fixed a couple of typos in commit message,
> patches unchanged. Original intro follows. ]
> 
> All -
> 
> This is small set of patches our team has had kicking around for a few versions
> internally that fixes tasks getting hung on shm_exit when there are many
> threads hammering it at once.
> 
> 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 :-)

> 
> Before applying this patchset, this test code will cause either hanging
> tracebacks or pthread out of memory errors.

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?

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.

In any case, I will take a closer look at the set.

Thanks,
Davidlohr

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