One use for reference-count biases... [LWN.net]

One use for reference-count biases...

Posted Jun 9, 2009 5:39 UTC (Tue) by neilbrown (subscriber, #359)
In reply to: One use for reference-count biases... by PaulMcKenney
Parent article: Linux kernel design patterns - part 1

Yes, I wouldn't be surprised if reference counts that are spread across multiple per-cpu variables would have very different trade-offs and hence different Patterns to the more traditional kinds!

I'm having trouble picturing exactly how the counters you describe would work (and particularly exactly what happens when the per-cpu counter hits zero) but it seems possible that the "one bit of information" that I claim a bias stores would, in this case, be the bit "Someone cares about a precise total value" and that one bit could conceivably be stored in a read-mostly cache line that could be easily shared among multiple processors.

So the code might look like:

   if (dec_and_test(per_cpu_counter))
         if (__test_bit(flag_name, &flag_variable))
               do some costly cross-cpu thing;

Is there any chance that would achieve the same result?

It may well be a case where you don't want to pay the price of an extra bit though.

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One use for reference-count biases...

Posted Jun 9, 2009 13:50 UTC (Tue) by PaulMcKenney (subscriber, #9624) [Link]

Hmmmm... I was thinking more in terms of something like the following:

preempt_disable(); if (per_cpu_counter > 0) per_cpu_counter++; else do some costly global-lock-and-variable thing preempt_enable();

But it has been a good ten years since I messed with this, so I should take another look at it.

In any case, I very much agree with your overall premise that higher-level primitives are a very great improvement over continually re-inventing the wheel, most especially for those wheels with a strong history of being re-invented badly!!!

One use for reference-count biases...

Posted Dec 23, 2011 5:30 UTC (Fri) by atiqure (guest, #81951) [Link]

hii.. i dnt know who r u but i need help... please tell me something about how to develop a kernel without using any source i just want to use programming language like c,c++,java and assembly language can i do that with these languages ..if yes then please tell me from where to start and how ...pls help it will be christmas gift for me from you ...