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Re: condvar performance in .59 vs .60

Boris Kolpackov wrote:

> First variation of the test runs about 10 times longer on 60 
> compared to 59. At the same time decrease in rate is less than
> 3 times.

This is an extreme case and probably not a very realistic pattern.  The
worst slowdown I've seen was, I think, 5%.  Yes, some things got slower
because the kernel is now, on SMP machines, too efficient and releases
the waiter so fast, that it runs into the blocked internal mutex.
Resulting in more context switches etc.

The method used until 0.59 wasn't reliable.  It should in theory work,
but in some extreme cases, under very high load, it locked up.  I think
if you'd run more threads and on a >= 4p machine you'd see lockups, too.

I'll spend more time on the requeue problem later, when I have a bit
more time.  It's more important to have the implementation correct now.
 And as I said, the performance degregation is very much lower in the
real world applications I've seen.

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