Re: [BUG] scheduler: first timeslice of the exiting thread

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 07 Apr 2007 16:31:39 +0900 Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com> wrote:

> When I was examining the following program ...
> 
>   1. There are a large amount of small jobs takes several msecs,
>      and the number of job increases constantly.
>   2. The process creates a thread or a process per job (I examined both
>      the thread model and the process model).
>   3. Each child process/thread does the assigned job and exit immediately.
> 
> ... I found that the thread model's latency is longer than proess
> model's one against my expectation. It's because of the current
> sched_fork()/sched_exit() implementation as follows:
> 
>   a) On sched_fork, the creator share its timeslice with new process.
>   b) On sched_exit, if the exiting process didn't exhaust its first
>      timeslice yet, it gives its timeslice to the parent.
> 
> It has no problem on the process model since the creator is the parent.
> However, on the thread model, the creator is not the parent, it is same
> as the creator's parent. Hence, on this kind of program, the creator
> can't retrieve shared timeslice and exausts its timeslice at a rate of
> knots. In addition, somehow, the parent (typically shell?) gets extra
> timeslice.
> 
> I believe it's a bug and the exiting process should give its timeslice
> to the creator. Now I have some patch plan to fix this problem as follow:
> 
>  a) Add the field for the creator to task_struct. It needs extra memory.
>  b) Doesn't add extra field and have thread's parent the creater, which is
>     same as process creation. However it has many side effects, for example,
>     we also need to change sys_getppid() implementation.
> 
> What do you think? Any comments are welcome.

This comes at an awkward time, because we might well merge the
staircase/deadline work into 2.6.22, and I think it rewrites the part of
the scheduler which is causing the problems you're observing.

Has anyone verified that SD fixes this problem and the one at
http://lkml.org/lkml/2007/4/7/21 ?