Re: [ATTEND] Attendance request

[Ted Ts'o <tytso@mit.edu>]

On Thu, Feb 02, 2012 at 03:51:03PM -0500, Zach Brown wrote:
> 
> Well, for that one blocking case in the submission path.
> 
> I certainly understand the desire to just make things less painful in
> this case, but I do wonder if the long-term win is to sink resources
> into trying to get threads executings async op so that the code paths
> don't have to be butchered into state machines around each potential
> blocking op.

I only intend to do this for reads and non-allocating writes; in those
cases, it's actually pretty easy.  And if everything is already in
cache, it's a lot easier to make things fast and not require diving
into workqueue or some other kernel thread.

My concern with just stuffing every single io_submit into a kernel
thread is the overhead involved, especially if there is a high rate of
AIO submission (think high-speed PCIe-attached flash).  Either you
will end up blocking waiting for a kernel thread to be available, or
you will burn a huge amount of kernel memory for the thread stacks if
nothing else.

						- Ted