Subj : Re: Lock Free -- where to start
To   : comp.programming.threads
From : chris noonan
Date : Thu Oct 06 2005 01:54 pm


David Schwartz wrote:
> > If locks are efficient, why does the technique called I/O Completion
> > Port exist?
> I don't understand what one has to do with the other. IOCP is about
> optimizing the number of concurrent threads without the risk of thread
> starvation.

But you are assuming that the optimum number of concurrent threads
is one per processor! (OK, I know IOCP sometimes schedules more).
That is only true if the performance of a large number of threads
would be destroyed by locks.
(I will have more to say on the IOCP fraud in a separate topic.)

> > Now introduce a lock, contended for by the threads,
> > such that the lock is held for 1% of a thread's
> > running time. Every hundredth timeslice, a thread
> > is switched out when it is holding the lock. During
> > the subsequent 20ms timeslice, all the other threads
> > run up against the lock, leading to 999 context
> > switches before the original thread gets the processor
> > again. Accordingly, every hundred timeslices there are
> > 1099 switches instead of 100, an eleven-fold increase.
> > That *is* going to sap performance.
>     Umm, no. Some thread will get the lock and unless the thread library is
> poorly designed, it will be allowed to do useful work at full speed without
> context switches. If other threads want the lock, they'll have to wait.

I am sure you are missing the point. Read the analysis again.
It is bound to happen that occasionally a thread switch (caused
asynchronously by a timer interrupt) will deschedule a thread
in the middle of some critical section thingy. No other thread
(in a homogeneous collection of threads running the same code path)
can thereafter get significant work done UNTIL the original thread
leaves the critical section. The scheduling algorithm in Windows NT
(for example) ensures that is not before 999 further switches have
been incurred. What operating system is it that allows a thread
to run indefinitely?

Chris

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