Fragmentation: Both Size and Lifetime

By: dmcq (dmcq.delete@this.fano.co.uk), June 30, 2022 11:12 am
Room: Moderated Discussions
⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on June 30, 2022 10:09 am wrote:
> Mark Roulo (nothanks.delete@this.xxx.com) on June 30, 2022 8:25 am wrote:
> > Foo_ (foo.delete@this.nomail.com) on June 30, 2022 7:43 am wrote:
> > > Adrian (a.delete@this.acm.org) on June 30, 2022 5:36 am wrote:
> > > >
> > > > In the memory allocators used by individual processes it is possible to completely avoid
> > > > memory fragmentation, e.g. by using separate memory pools for different object sizes.
> > >
> > > That doesn't make sense. Memory fragmentation is typically caused by differing
> > > *lifetimes* of heap allocations, not by differing object sizes.
> > >
> >
> > It is both.
> >
> > All the allocations the same size (e.g. 1KB) mean there is no
> > fragmentation because if you have enough memory for all your
> > objects then there will be a slot. You won't have a situation
> > where you need 1KB, have 5KB, but the 5KB is unusable because it
> > is in 100 discontiguous 50B pieces.
> >
> > Alternately, if one wants to allocate a large number of randomly
> > sized objects and then free them all at once (and you know this ...)
> > then you can avoid fragmentation by simply allocating "from the end"
> > and freeing by "resetting the 'next'" pointer.
> >
> > The intersection of different sizes and different lifetimes (and
> > not knowing them up front ...) creates the fragmentation.
>
> Ok ... so, in my not-so-humble-this-time opinion, both mentioned explanations about the "alleged" primary
> cause of memory fragmentation are actually invalid. The real/actual reason for memory fragmentation is
> the impossibility of computing the time-order of deallocations _before_ those deallocations are actually/physically
> executed. In cases in which it is possible - at the _allocation_ point - to compute the _deallocation_
> order, memory fragmentation either does not occur or does not need to occur (unless, of course, the programmer
> decides for fragmentation to occur on purpose because of some strange reason).
>
> The viewpoint that "memory fragmentation is caused by differences in sizes of objects" and the viewpoint that
> "memory fragmentation is caused by differing lifetimes of objects" are both limited/restricted viewpoints. The
> latter viewpoint is in my opinion slightly more accurate, but only slightly. The viewpoint that "there is some
> 'fundamental link' between [memory fragmentation] and [having multiple allocation pools instead of a single
> pool, or in reverse, having a single pool instead of multiple pools]" is a limited viewpoint as well.
>
> In summary: If the _deallocation_ order was computable in advance in all cases, then it would be
> possible to always _allocate_ memory in a way that is optimized in respect to the ordering.
>
> -atom

Would it? That definitely sounds like something that in Wikipedia would have citation neeeded pout aftyer it.
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              Linear Address Spaces: Unsafe at any speed [article]2022/07/01 10:41 PM
    Linear Address Spaces: Unsafe at any speed [article]Foo_2022/06/30 07:43 AM
      Fragmentation: Both Size and LifetimeMark Roulo2022/06/30 08:25 AM
        Fragmentation: Both Size and Lifetime2022/06/30 10:09 AM
          Fragmentation: Both Size and Lifetimedmcq2022/06/30 11:12 AM
          Fragmentation: Both Size and LifetimeBrendan2022/06/30 04:08 PM
            Fragmentation: Both Size and Lifetime2022/07/02 03:12 AM
              Fragmentation: Both Size and LifetimeBrendan2022/07/02 01:56 PM
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                      Fragmentation: Both Size and LifetimeBrendan2022/07/04 04:21 PM
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