[sheepdog] [PATCH] sheep: introduce 'short thread' to worker threads

[Liu Yuan]

On 06/29/2012 04:44 PM, MORITA Kazutaka wrote:
> Does the approach really show better performance and scalability than
> the current workqueue model with your short thread patch?  I'd like to
> see the performance number before accepting the change.

The real benefit is that we don't need pre-set any number of threads,
because we don't assume cluster nodes number and we just dynamically add
nodes in on demand. Presetting will for sure constraint cluster
scalability and statical setting will also waste resource when node is
not busy. And dynamic threads work on demand and no waste for any
conditions.

The performance number is not that relevant:
  1 it boils down to two pthread call efficient comparison:
pthread_cond_signal() and pthread_create(), both is heavily affected by
system resource usage at the time of being calling and kernel's scheduling.
  2 For a static code review, we can even see that short thread creation
won't need to hold wi->pending_lock at all and is scheduled to run once
is created (don't need notification and another lock holding). So I
think short threads performance might be slightly better than previous
statically work queue especially for concurrent coherent requests of
gateway and IO.

Also we can see that both gateway and io threads exhibit 'on demand'
characteristics: we need much more threads when system is busy and very
few when is not, so we need on-demand threads implementation for that too.

Thanks,
Yuan