locks and kernel randomness...

[John Baldwin]

On Wednesday, February 25, 2015 10:56:59 AM Konstantin Belousov wrote:
> The cost of the proposed patch, of course, is not the several
> thousands of instructions in the rebalance. The problem with it is the
> introduction of the new spinlock, which will be used in many places
> after the introduction. The cost of the new and often used spinlock is
> the increase of both interrupt latency and interrupt handler jitter and
> cpu switch jitter.
> 
> So neither buildworld timing, nor network throughput are adequate
> to estimate the change.  It is system unresponsivness and loss of
> the realtime behaviour up to some degree.
> 
> I thought that it was obvious, at least after spinlocks were mentioned,
> but apparently it is not, since proposals to measure the patch effect
> by benchmarking buildworld or passing the traffic are made.

+1

The only thing I will add is that in general this makes the system more 
fragile and complex as well.  Please just stay with a regular mutex and change 
the scheduler to not use random() (which seems to be in progress?).  I'm not 
sure why we needed the extra 40 messages in this thread after that was 
effectively said the first time.

-- 
John Baldwin