Re: fine points of dynamic memory allocation, not 80286 protected mode

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Sujet : Re: fine points of dynamic memory allocation, not 80286 protected mode
De : tr.17687 (at) *nospam* z991.linuxsc.com (Tim Rentsch)
Groupes : comp.arch
Date : 18. Oct 2024, 15:12:51
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <86cyjxwlcs.fsf@linuxsc.com>
References : 1 2 3 4 5
User-Agent : Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux)
John Levine <johnl@taugh.com> writes:

According to Tim Rentsch  <tr.17687@z991.linuxsc.com>:
>
Once there is a way to get additional memory from whatever underlying
environment is there, malloc() and free() can be implemented (and I
believe most often are implemented) without needing to compare
pointers.  Note:  pointers can be tested for equality without having
to compare them relationally, and testing pointers for equality is
well-defined between any two pointers (which may need to be converted
to 'void *' to avoid a type mismatch).
>
I suppose it's possible, but the versions I know keep free lists
and consolidate adjacent free chunks which requires comparing the
pointers.

It might seem that way, but it isn't so.  There is a fairly
straightforward scheme -- using a single address-sized cell
before each memory block -- that consolidates adjacent free
chunks and maintains free lists, without needing to compare
pointers (and in fact doesn't use pointers at all except for the
free lists).  Doing a malloc() needs to search an appropriate
free list for an adequately large free block, and is constant
time thereafter;  doing a free() uses constant time, including
consolidation of adjacent free chunks.  For an implementation
with 8-byte addresses, this can be done with a grain size of 16
bytes and a minimum of 32-byte blocks (of which 24 bytes can be
used by the client).  Coincidentally (or not) that matches the
sizes I see in tests done on an Ubuntu Linux system (any size
less than 25 uses 32 bytes, any size less than 41 uses 48 bytes,
etc).

The key insight is to use the preceding memory word to hold the
size of this block plus two bits: bit A is one if this block is
not free, and bit B is one if the previous block is not free.  If
bit B is zero (so the previous block is free) then the last word
of the previous block holds the size of that block.  Free lists
are maintained using the first two words in each free block (of
course plus the list headers, which is a small fixed overhead).
This information is enough to combine adjacent free blocks when a
free() is done.

Date Sujet#  Auteur
16 Apr 24 * Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)237Lawrence D'Oliveiro
16 Apr 24 `* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)236David Brown
16 Apr 24  +- Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)1MitchAlsup1
26 May 24  +- Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)1MitchAlsup1
1 Oct 24  `* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)233MitchAlsup1
1 Oct 24   `* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)232Thomas Koenig
1 Oct 24    +* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)225MitchAlsup1
2 Oct 24    i+* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)223Brett
3 Oct 24    ii`* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)222Lawrence D'Oliveiro
3 Oct 24    ii +- Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)1Brett
3 Oct 24    ii +- Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)1Anton Ertl
3 Oct 24    ii `* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)219David Brown
3 Oct 24    ii  `* Byte ordering (was: Whether something is RISC or not)218Anton Ertl
3 Oct 24    ii   +- Re: Byte ordering (was: Whether something is RISC or not)1David Brown
4 Oct 24    ii   +* Re: Byte ordering (was: Whether something is RISC or not)215Lawrence D'Oliveiro
4 Oct 24    ii   i+- Re: Byte ordering1Lynn Wheeler
4 Oct 24    ii   i+* Re: Byte ordering (was: Whether something is RISC or not)211David Brown
4 Oct 24    ii   ii`* Re: Byte ordering (was: Whether something is RISC or not)210Anton Ertl
4 Oct 24    ii   ii +* Re: Byte ordering5BGB
5 Oct 24    ii   ii i`* Re: Byte ordering4MitchAlsup1
5 Oct 24    ii   ii i +* Re: Byte ordering2BGB
5 Oct 24    ii   ii i i`- Re: Byte ordering1Lawrence D'Oliveiro
5 Oct 24    ii   ii i `- Re: Byte ordering1Lawrence D'Oliveiro
5 Oct 24    ii   ii +* Re: Byte ordering (was: Whether something is RISC or not)13Lawrence D'Oliveiro
5 Oct 24    ii   ii i`* Re: Byte ordering (was: Whether something is RISC or not)12Brett
5 Oct 24    ii   ii i `* Re: Byte ordering (was: Whether something is RISC or not)11Anton Ertl
5 Oct 24    ii   ii i  `* Re: Byte ordering (was: Whether something is RISC or not)10Michael S
6 Oct 24    ii   ii i   +- Re: Byte ordering1Terje Mathisen
6 Oct 24    ii   ii i   `* Re: Byte ordering (was: Whether something is RISC or not)8Brett
7 Oct 24    ii   ii i    `* Re: Byte ordering (was: Whether something is RISC or not)7Lawrence D'Oliveiro
7 Oct 24    ii   ii i     `* Re: Byte ordering (was: Whether something is RISC or not)6Brett
7 Oct 24    ii   ii i      `* Re: Byte ordering (was: Whether something is RISC or not)5Michael S
7 Oct 24    ii   ii i       +* Re: Byte ordering2Stefan Monnier
7 Oct 24    ii   ii i       i`- Re: Byte ordering1Michael S
7 Oct 24    ii   ii i       `* Re: Byte ordering (was: Whether something is RISC or not)2Lawrence D'Oliveiro
8 Oct 24    ii   ii i        `- Re: Byte ordering1Terje Mathisen
6 Oct 24    ii   ii `* Re: Byte ordering191David Brown
6 Oct 24    ii   ii  `* Re: Byte ordering190Anton Ertl
6 Oct 24    ii   ii   `* Re: Byte ordering189John Dallman
7 Oct 24    ii   ii    +* Re: Byte ordering20Lawrence D'Oliveiro
8 Oct 24    ii   ii    i`* Re: Byte ordering19John Dallman
9 Oct 24    ii   ii    i +- VMS/NT memory management (was: Byte ordering)1Stefan Monnier
15 Oct 24    ii   ii    i +* Re: Byte ordering2Lawrence D'Oliveiro
15 Oct 24    ii   ii    i i`- Re: Byte ordering1MitchAlsup1
15 Oct 24    ii   ii    i `* Re: Byte ordering15Lawrence D'Oliveiro
15 Oct 24    ii   ii    i  +* Re: Byte ordering3Michael S
15 Oct 24    ii   ii    i  i+- Re: Byte ordering1John Dallman
18 Oct 24    ii   ii    i  i`- Re: Byte ordering1Lawrence D'Oliveiro
15 Oct 24    ii   ii    i  +* Re: Byte ordering9John Dallman
16 Oct 24    ii   ii    i  i+* Re: Byte ordering7George Neuner
16 Oct 24    ii   ii    i  ii`* Re: Byte ordering6Terje Mathisen
16 Oct 24    ii   ii    i  ii `* Re: Byte ordering5David Brown
17 Oct 24    ii   ii    i  ii  +* Re: Byte ordering2George Neuner
17 Oct 24    ii   ii    i  ii  i`- Re: Byte ordering1David Brown
17 Oct 24    ii   ii    i  ii  `* Re: clouds, not Byte ordering2John Levine
17 Oct 24    ii   ii    i  ii   `- Re: clouds, not Byte ordering1David Brown
18 Oct 24    ii   ii    i  i`- Re: Byte ordering1Lawrence D'Oliveiro
16 Oct 24    ii   ii    i  `* Re: Byte ordering2Paul A. Clayton
18 Oct 24    ii   ii    i   `- Re: Microkernels & Capabilities (was Re: Byte ordering)1Lawrence D'Oliveiro
7 Oct 24    ii   ii    `* 80286 protected mode168Anton Ertl
7 Oct 24    ii   ii     +* Re: 80286 protected mode5Lars Poulsen
7 Oct 24    ii   ii     i`* Re: 80286 protected mode4Terje Mathisen
7 Oct 24    ii   ii     i +- Re: 80286 protected mode1Michael S
7 Oct 24    ii   ii     i `* Re: 80286 protected mode2Lawrence D'Oliveiro
8 Oct 24    ii   ii     i  `- Re: 80286 protected mode1Terje Mathisen
7 Oct 24    ii   ii     +* Re: 80286 protected mode3Brett
7 Oct 24    ii   ii     i`* Re: 80286 protected mode2Michael S
7 Oct 24    ii   ii     i `- Re: 80286 protected mode1Brett
7 Oct 24    ii   ii     +- Re: 80286 protected mode1Lawrence D'Oliveiro
8 Oct 24    ii   ii     +* Re: 80286 protected mode152MitchAlsup1
8 Oct 24    ii   ii     i+* Re: 80286 protected mode4Lawrence D'Oliveiro
8 Oct 24    ii   ii     ii`* Re: 80286 protected mode3MitchAlsup1
9 Oct 24    ii   ii     ii +- Re: 80286 protected mode1David Brown
15 Oct 24    ii   ii     ii `- Re: 80286 protected mode1Lawrence D'Oliveiro
8 Oct 24    ii   ii     i`* Re: 80286 protected mode147Anton Ertl
8 Oct 24    ii   ii     i +- Re: 80286 protected mode1Robert Finch
9 Oct 24    ii   ii     i `* Re: 80286 protected mode145David Brown
9 Oct 24    ii   ii     i  +* Re: 80286 protected mode79MitchAlsup1
9 Oct 24    ii   ii     i  i`* Re: 80286 protected mode78David Brown
9 Oct 24    ii   ii     i  i `* Re: 80286 protected mode77Stephen Fuld
10 Oct 24    ii   ii     i  i  +* Re: 80286 protected mode2MitchAlsup1
10 Oct 24    ii   ii     i  i  i`- Re: 80286 protected mode1David Brown
10 Oct 24    ii   ii     i  i  +- Re: 80286 protected mode1David Brown
11 Oct 24    ii   ii     i  i  `* Re: 80286 protected mode73Tim Rentsch
15 Oct 24    ii   ii     i  i   `* Re: 80286 protected mode72Stefan Monnier
15 Oct 24    ii   ii     i  i    +* Re: 80286 protected mode30MitchAlsup1
16 Oct 24    ii   ii     i  i    i+* Re: 80286 protected mode25MitchAlsup1
16 Oct 24    ii   ii     i  i    ii+* Re: C and turtles, 80286 protected mode13John Levine
16 Oct 24    ii   ii     i  i    iii+* Re: C and turtles, 80286 protected mode7MitchAlsup1
16 Oct 24    ii   ii     i  i    iiii`* Re: C and turtles, 80286 protected mode6John Levine
17 Oct 24    ii   ii     i  i    iiii `* Re: C and turtles, 80286 protected mode5Thomas Koenig
20 Oct 24    ii   ii     i  i    iiii  `* Re: C and turtles, 80286 protected mode4Lawrence D'Oliveiro
20 Oct 24    ii   ii     i  i    iiii   `* Re: C and turtles, 80286 protected mode3George Neuner
22 Oct 24    ii   ii     i  i    iiii    `* Re: C and turtles, 80286 protected mode2Tim Rentsch
22 Oct 24    ii   ii     i  i    iiii     `- Re: C and turtles, 80286 protected mode1George Neuner
16 Oct 24    ii   ii     i  i    iii+- Re: C and turtles, 80286 protected mode1David Brown
16 Oct 24    ii   ii     i  i    iii`* Re: C and turtles, 80286 protected mode4Paul A. Clayton
17 Oct 24    ii   ii     i  i    iii +- Re: C and turtles, 80286 protected mode1David Brown
20 Oct 24    ii   ii     i  i    iii `* Re: C and turtles, 80286 protected mode2Lawrence D'Oliveiro
20 Oct 24    ii   ii     i  i    iii  `- Re: C and turtles, 80286 protected mode1Paul A. Clayton
16 Oct 24    ii   ii     i  i    ii+* Re: 80286 protected mode7Thomas Koenig
17 Oct 24    ii   ii     i  i    ii+* Re: 80286 protected mode3George Neuner
17 Oct 24    ii   ii     i  i    ii`- Re: 80286 protected mode1Tim Rentsch
16 Oct 24    ii   ii     i  i    i+* Re: 80286 protected mode3David Brown
17 Oct 24    ii   ii     i  i    i`- Re: 80286 protected mode1Tim Rentsch
16 Oct 24    ii   ii     i  i    `* Re: 80286 protected mode41David Brown
9 Oct 24    ii   ii     i  +* Re: 80286 protected mode51Thomas Koenig
13 Oct 24    ii   ii     i  `* Re: 80286 protected mode14Anton Ertl
8 Oct 24    ii   ii     `* Re: 80286 protected mode6John Levine
6 Oct 24    ii   i`* Re: Byte ordering (was: Whether something is RISC or not)2Michael S
4 Oct 24    ii   `- Re: Byte ordering (was: Whether something is RISC or not)1John Dallman
2 Oct 24    i`- Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)1Thomas Koenig
2 Oct 24    +* Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)5David Schultz
3 Oct 24    `- Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)1Lawrence D'Oliveiro

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