Hi,

Adrian (a.delete@this.acm.org) on July 3, 2022 10:04 pm wrote:
> Foo_ (foo.delete@this.nomail.com) on July 3, 2022 2:45 am wrote:
> > Adrian (a.delete@this.acm.org) on July 1, 2022 9:58 pm wrote:
> > > Foo_ (foo.delete@this.nomail.com) on July 1, 2022 3:21 am wrote:
> > > > Adrian (a.delete@this.acm.org) on June 30, 2022 9:43 am wrote:
> > > > >
> > > > > When all the allocation and free requests are done from stacks, each stack corresponding
> > > > > to a certain size class, nothing ever changes from the point of view of fragmentation.
> > > >
> > > > This is delusional. Allocation may be done "from stacks", but given that deallocations
> > > > happen in random order (random from the POV of the allocator, of course), they
> > > > cannot be done in a simple stack-like (i.e. LIFO) fashion.
> > >
> > >
> > > When the size is variable, you are right.
> > >
> > > But when the size of a memory block is fixed (being used to satisfy any allocation
> > > requests between 2 thresholds), the allocations and deallocations are really
> > > done as LIFO from the stack containing the free memory blocks.
> >
> > You don't read what you reply to, do you?
> >
>
>
> Actually I have read carefully, but maybe I have not explained clearly enough.

As far as I can tell, all of the confusion is caused by different definitions of "fragmentation".

Specifically, you don't seem to distinguish between "fragmentation" and "negative consequences of fragmentation".

For example, your previous "Due to paging, which remaps the addresses, all pages become equivalent, it does not matter which are allocated at a request. This reduces the memory allocation problem for the OS to the simple case of allocations of fixed size, when there is no danger of memory fragmentation" could/should be rephrased as "paging allows both allocated physical memory and free physical memory to become extremely fragmented; but hides most of the negative consequences of fragmentation".

Of course paging doesn't avoid all negative consequences of fragmentation - e.g. if you want to transfer a larger amount of data to a device you have to split it into page sized pieces due to the fragmentation of allocated physical memory; and if you want to support "hot-remove" RAM or merge many free smaller pages back into a large free page then you end up needing a way to defragment part of physical memory.

In the same way; you think that a stack of fixed sized objects avoids fragmentation of the underlying (virtual) memory when it does not (and merely hides most of the negative consequences of fragmentation). E.g. with a stack of fixed sized objects; if you allocate 1 million objects (asking OS for more virtual memory when you run out) and then free every second object, you will be unable to return the unused underlying (virtual) memory back to the OS because it has become extremely fragmented.

In other words; from other people's perspective (mine), it seems like you're wrong, and it seems like you don't know what "fragmented" means.

- Brendan