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Re: Defined and undefined C pointer manipulation

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James Harris <james.harris.1@gmail.com> wrote:

(snip, someone wrote)
>> You still have the annoying problem that malloc needs somehow 
>> to remember the size of each allocated block.

(snip) 
> ISTM that most malloc implementations place their memory managment nodes 
> between the allocated memory spaces which would cause the alignment creep 
> you mention. It is also potentially fragile because 1) those spaces cannot 
> be protected, and 2) a pointer going just beyond where it should could lead 
> to corruption of a node.

One reason not to is the poor peformance of virtual memory systems.
 
> I came up with an idea for a memory allocator which stores all of its 
> metadata elsewhere (though I am not looking at impementing that just now). 
> It is a little more complex and wouldn't have such good free() performance. 
> free() normally just has to offset the pointer it is passed in order to find 
> the node but if not stored relative to the start of the allocated memory 
> space the node would need to be found. Storing the node-type data elsewhere, 
> though, does buy you quite a bit: greater security, the alignments you want 
> and often faster scanning for malloc to find space.

I know some put at least some of the data elsewhere for better
virtual memory reasons. If you follow a linked list through the
beginning of each allocated block, you have to page in that block.

I know some do it differently, but I don't remember how much.
You could put a linked list somewhere else, and a pointer to a
link in the list before each block, to make free fast. 
A hash table for free should also be pretty fast.
 
> Don't forget that alignment creep can be a good thing if it ends up 
> offsetting cache lines that are used together so perfect alignment can 
> sometimes be a bad thing - less of a problem now with CPUs which have 
> many-way caches.

I suppose, but you could do that even without the data before
each block. 

One that I have wondered about is web browsers and the data that
they keep for each tab or window.  Many perform poorly, doing a lot
of paging, as their memory use gets big. 

-- glen

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Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-26 18:35 +0100
  Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-26 12:04 -0700
    Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 19:11 +0100
      Re: Defined and undefined C pointer manipulation Barry Schwarz <schwarzb@dqel.com> - 2014-04-27 11:46 -0700
        Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 12:46 -0700
        Re: Defined and undefined C pointer manipulation Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-28 02:53 +0100
      Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 12:57 -0700
        Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 21:56 +0100
          Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 14:25 -0700
            Re: Defined and undefined C pointer manipulation glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-27 22:51 +0000
  Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-27 06:21 -0700
    Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-27 16:51 +0100
      Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-27 10:36 -0700
      Re: Defined and undefined C pointer manipulation Ian Collins <ian-news@hotmail.com> - 2014-04-28 11:40 +1200
  Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 19:23 +0100
    Re: Defined and undefined C pointer manipulation Richard Damon <Richard@Damon-Family.org> - 2014-04-27 14:56 -0400
      Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 20:52 +0100
        Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 13:16 -0700
    Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-27 20:55 +0100
      Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 21:20 +0100
        Re: Defined and undefined C pointer manipulation glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-27 22:39 +0000
        Re: Defined and undefined C pointer manipulation glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-27 22:43 +0000
        Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-28 00:13 +0100
    Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 13:01 -0700
    Re: Defined and undefined C pointer manipulation James Kuyper <jameskuyper@verizon.net> - 2014-04-27 22:07 -0400
    Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 00:47 -0700
      Re: Defined and undefined C pointer manipulation Ian Collins <ian-news@hotmail.com> - 2014-04-28 20:01 +1200
        Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 02:11 -0700
          Re: Defined and undefined C pointer manipulation Ian Collins <ian-news@hotmail.com> - 2014-04-28 22:38 +1200
            Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 04:50 -0700
      Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-28 09:22 +0100
        Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 05:31 -0700

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