Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose number of tasks running on cpu

[Tim Chen]

On Mon, 2014-07-14 at 18:14 +0200, Peter Zijlstra wrote:
> On Mon, Jul 14, 2014 at 09:10:14AM -0700, Tim Chen wrote:
> > On Mon, 2014-07-14 at 12:16 +0200, Peter Zijlstra wrote:
> > > On Fri, Jul 11, 2014 at 01:33:04PM -0700, Tim Chen wrote:
> > > > This function will help a thread decide if it wants to to do work
> > > > that can be delayed, to accumulate more tasks for more efficient
> > > > batch processing later.
> > > > 
> > > > However, if no other tasks are running on the cpu, it can take
> > > > advantgae of the available cpu cycles to complete the tasks
> > > > for immediate processing to minimize delay, otherwise it will yield.
> > > 
> > > Ugh.. and ignore topology and everything else.
> > > 
> > > Yet another scheduler on top of the scheduler.
> > > 
> > > We have the padata muck, also only ever used by crypto.
> > > We have the workqueue nonsense, used all over the place
> > > And we have btrfs doing their own padata like muck.
> > > And I'm sure there's at least one more out there, just because.
> > > 
> > > Why do we want yet another thing?
> > > 
> > > I'm inclined to go NAK and get people to reduce the amount of async
> > > queueing and processing crap.
> > 
> > The mult-buffer class of crypto algorithms is by nature
> > asynchronous.  The algorithm gathers several crypto jobs, and
> > put the buffer from each job in a data lane of the SIMD register.
> > This allows for parallel processing and increases throughput.
> > The gathering of the crypto jobs is an async process and
> > queuing is necessary for this class of algorithm.
> 
> How is that related to me saying we've got too much of this crap
> already?

I was trying to explain why the algorithm is implemented this way
because of its batching nature.

There is a whole class of async algorithm that can provide
substantial speedup by doing batch processing and uses workqueue.
The multi-buffer sha1 version has 2.2x speedup over existing
AVX2 version, and can have even more speedup when AVX3 
comes round.  Workqueue is a natural way to implement 
this.  I don't think a throughput speedup of 2.2x is "crap".

We are not inventing anything new, but ask for a 
very simple helper function to know if there's something else
running on our cpu to help us make a better decision 
of whether we should flush the batched jobs immediately.

And also asynchronous crypto interface is already used substantially
in crypto and has a well established infrastructure.  

Thanks.

Tim


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