Re: Approximate structure-allocation limit problem improvement

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Dec 16, 2015 at 02:21:45PM +0100, Colin Pitrat wrote:
> Hello,
> 
> in chapter 5 (Counting), in the paragraph concerning "Approximate
> structure-allocation limit problem", it is said:
> 
> "Similarly, sub_count() can fail even when the aggregate value of the
> counter is nowhere near zero. In many cases, this is unacceptable."
> 
> In the case of the Quick Quiz 5.3 question, it's true that it's quite
> a problem if it means freeing the structure fails because the counter
> cannot be updated ! And freeing it without updating the counter means
> the counter shifts from reality.
> 
> However, the very existence of the structure is a guarantee that the
> real (total) counter value cannot be 0 or less than delta, so couldn't
> we imagine to always succeed the sub_count operation by keeping a
> global negative offset ? In the slow path, after globalize_count, if
> the global count is lower than delta then we set it to 0 and we
> increment the negative offset by what remains.
> 
> On the next slow path operation during a add_count, this negative
> offset can be removed (or reduced) by reducing the local counter of
> the thread by the same value.
> 
> This improvement over the proposed solution make it an acceptable
> answer for the quick quiz 5.3 whereas it isn't without.
> 
> What do you think ?
> Should I spend a bit of time writing a paragraph and a code sample about it ?

I must confess that I have read this message a few times, and still don't
understand what you are getting at.  Part of my trouble is that I am
not sure why you are comparing against zero -- the structure-allocation
problem only needs to compare against the limit.  But you might mean
zero after offset, or you might be talking about some related problem.

So perhaps a paragraph and sample code would help me understand the
problem you are looking at and your example solution.

							Thanx, Paul