[haiku-gsoc] Re: XSI semaphore implamentation patch #2

On 2008-07-30 at 02:01:47 [+0200], Salvatore Benedetto <emitrax@xxxxxxxxx> 
wrote:
> 2008/7/29 Ingo Weinhold <ingo_weinhold@xxxxxx>:
> >> Because as I said, the performance issues happens only on process exit 
> >> and
> >> from
> >> an user point of view it shouldn't be a problem.
> >
> > Not sure what to say. You wrote yourself in your previous mail that SETVAL
> > and SETALL are affected (BTW, the latter is even very suboptimally
> > implemented and thus O(m*n) with m = number of sems in the set and n = 
> > total
> > number of undo entries). And as I wrote in my previous mail RMID should be
> > affected in the same way. So the performance issues when a lot of 
> > semaphores
> > are used are definitely not limited to program exit only.
> 
> Exactly. It also affect the performances of initialization, as that's
> what SETVAL and
> SETALL are. Theorically during inizialization there won't be any
> process waiting on
> it, nor there be a sem_undo request queued.
> 
> The other design I started with, required a private list in every
> XsiSemaphore, so that
> when SETALL or SETVAL was called, all it has to do was iterate through
> the list and
> set the sem_undo value to zero. Despite improving the inizialization
> performance, it
> would also require to add another list to the team to keep an object
> per semaphore
> used with SEM_UNDO. This list would be accessed at exit time in the
> very same manner
> the XsiSemaphore private one is. This design though, add the
> complexity of keeping the
> two list synchronized, plus something else I don't recall right now

Managing too lists is not really more complex than managing one list and a 
counter. You basically have to replace the counter increments/decrements by 
list adds/removes. The only additional complexity is the per team structure, 
but that is needed anyway to keep track of private semaphore sets, that 
should be deleted on team exit/exec*().

I would go with an undo entry per semaphore set and team, BTW.

> Anyway, I decided to choose semplicity as I though that the
> performances lost on exit
> and initialization won't be a big problem. Theorically there won't
> even be a process with
> requested sem_undo at inizialization, as theorically, SETVAL and
> SETALL are the very
> first operation done on a set.

It doesn't make a difference when SETVAL/SETALL are called or whether there 
are undo entries for these semaphores, since you unconditionally iterate 
through the whole global undo entry list in any case.

CU, Ingo

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