Re: [PATCH, RFC] simplify writeback thread creation

[Artem Bityutskiy <dedekind1@gmail.com>]

Thanks Christoph,

I'll try to test it in my environment.

On Thu, 2010-07-08 at 00:52 +0200, Christoph Hellwig wrote:
> Currently the per-bdi writeback thread is only created when there is
> dirty any dirty data on the BDI, and it lazy exists when it's been
> unused for some time.
> 
> This leads to some very complex code, and the need to keep a forker
> thread around.

I'd add here a comment that due to the complexity it is not bug-free -
e.g., I pointed to the race between thread suicide and wake-up.

Also I'd add a comment that it leads to more thread types waking up
periodically and consuming power. With just one thread type it is easier
to fix this issue.

> This patch removes all this code and simply creates the thread as part
> of the bdi registration.  The downside is that we use up ressoures
> for possible unused devices, although that overhead is rather low,
> with 8k kernel stack size on x86 and few other, even smaller ressources.

s/ressources/resources/
:-)

On the other hand, there was a message that process IDs are precious
resource on large NUMA systems. So, less suicides, more free IDs :-) But
this is just a minor note.

> If the overhead is still considered too much I can look into starting
> the thread explicitly instead of as part of the bdi registration, but
> that will require a bit of code complexity, too.

Well, the threads can exit, then we need:

1. when adding bdi works and the thread exited, forr it.
2. the same should be done in __mark_inode_dirty - if the bdi thread is
dead, create it.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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