Re: Memory management and custom allocator

From	Phil Carmody <thefatphil_demunged@yahoo.co.uk>
Newsgroups	comp.lang.c
Subject	Re: Memory management and custom allocator
Date	2012-01-07 05:55 +0200
Organization	A noiseless patient Spider
Message-ID	<87obugw4i7.fsf@bazspaz.fatphil.org> (permalink)
References	(2 earlier) <jdqjtf$bqd$1@reader1.panix.com> <jdsact$nr6$1@dont-email.me> <je2hno$ac$2@reader1.panix.com> <je2uhu$6s5$1@dont-email.me> <Eh7Nq.224866$cr3.132144@newsfe03.ams2>

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"BartC" <bc@freeuk.com> writes:
> "Eric Sosman" <esosman@ieee-dot-org.invalid> wrote in message
> news:je2uhu$6s5$1@dont-email.me...
> > On 1/4/2012 4:52 PM, Joe keane wrote:
> >> In article<jdsact$nr6$1@dont-email.me>,
> >> Eric Sosman<esosman@ieee-dot-org.invalid>  wrote:
> >>>      If you have a forty-four-byte struct and allocate sixty-four
> >>> bytes to hold it, how have you avoided *either* sort of "waste?"
> >>
> >> If the cache line is 16 bytes, we know that each object will take only
> >> three lines; the fourth is never used [it may be worse with the VM].
> >> For this case 48 would work just as well (the compiler will do that for
> >> you if it thinks 8-byte alignment is a good idea).  But then if the
> >> cache line is 32 bytes, same argument applies.
> >
> >     All I can say is that you and I have divergent notions of
> > efficiency.  I'm thinking "I can run over the entire array with
> > just N*44/C cache misses; Joe would prefer N*64/C."  That is, I
> > think you'll incur 46% more cache misses than I will.  Tell me
> > again, please, what I have wasted by using 31% fewer cache line
> > loads than you would?
> 
> I think he means that the last 16-bytes of the 64 might never be accessed,
> so will never be fetched from memory in the first place; there is no cache
> miss.

Were there to be no hits on google for "64 bytes (one cache line)", then 
you might have a point. The fact that most of such hits refer to the single
most popular general purpose computer architecture in the world diminishes
your point significantly. Those bytes are being pulled into the cache whether
you access them or not.
 
> (That does depend on how the 64-bytes are used; if the whole struct is
> copied, the compiler might access all 64-bytes anyway.)
> 
> And if the cache blocks are 32-bytes, then it doesn't matter.

Nonsense. An adjacent bunch of Eric's structures being pulled into the 
working set will displace fewer cache lines than the same number of your
structs. It's only if the cache lines are 16 bytes (i.e. not any of the
half dozen architectures I've worked with in recent years) that both
will cause 48 bytes to be churned.

> >     I'm also thinking "I need N*44/P pages for my array; Joe would
> > rather use N*64/P."  That is, I think your working set size will
> > be 46% larger than mine, your likelihood of taking a page fault
> > will be 46% greater than mine.  Tell me again, please, what have
> > I wasted by using 31% fewer memory pages?
> 
> No one was advocating wasting 20 bytes, but finding a use for them, perhaps
> moving data there that would have been stored elsewhere.

How about the "use" of "still being available for subsequent allocations"?

> >     It's an odd sort of "waste" that spends fewer resources.
> 
> If you had a heavily used struct, and it turned out it had 65 bytes, that
> wouldn't bother you any? You wouldn't try and get it down to 64?

Straw man.

Phil
-- 
Unix is simple. It just takes a genius to understand its simplicity
-- Dennis Ritchie (1941-2011), Unix Co-Creator

Thread

[Slightly OT] Memory management and custom allocator FtM <fmassei@gmail.com> - 2011-12-31 06:56 -0800
  Re: [Slightly OT] Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-31 11:08 -0500
    Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2011-12-31 08:28 -0800
  Re: [Slightly OT] Memory management and custom allocator Johann Klammer <klammerr@NOSPAM.aon.at> - 2011-12-31 17:39 +0100
    Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2011-12-31 08:55 -0800
      Re: Memory management and custom allocator Johann Klammer <klammerr@NOSPAM.aon.at> - 2011-12-31 19:01 +0100
        Re: Memory management and custom allocator Kaz Kylheku <kaz@kylheku.com> - 2011-12-31 18:31 +0000
  Re: [Slightly OT] Memory management and custom allocator Kaz Kylheku <kaz@kylheku.com> - 2011-12-31 17:24 +0000
    Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2011-12-31 09:52 -0800
      Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-31 14:30 -0500
      Re: Memory management and custom allocator Markus Wichmann <nullplan@gmx.net> - 2012-01-01 12:21 +0100
        Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-01 12:54 +0000
          Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-01 09:03 -0500
            Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-01 16:23 +0000
            Re: Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-01 21:41 +0000
              Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-02 08:10 -0500
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-02 14:07 +0000
                Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-02 09:47 -0500
                Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-02 08:36 -0800
                Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-02 13:43 -0500
                Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-02 11:38 -0800
                Re: Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-03 13:14 +0000
                Re: Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-03 06:07 -0800
                Re: Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-05 17:06 -0800
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-02 21:36 +0000
                Re: Memory management and custom allocator Kaz Kylheku <kaz@kylheku.com> - 2012-01-02 21:44 +0000
                Re: Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-02 23:10 -0800
                Re: Memory management and custom allocator Keith Thompson <kst-u@mib.org> - 2012-01-02 13:42 -0800
                Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-02 17:38 -0500
                Re: Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-02 21:45 +0000
                Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-02 18:19 -0500
                Re: Memory management and custom allocator Keith Thompson <kst-u@mib.org> - 2012-01-02 15:42 -0800
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-02 23:59 +0000
                Re: Memory management and custom allocator Kleuske <kleuske@nowhere.net> - 2012-01-03 11:38 +0000
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-03 12:49 +0000
                Re: Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-04 23:42 +0000
                Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-04 20:23 -0500
                Re: Memory management and custom allocator Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-01-05 23:31 +0200
                Re: Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-04 21:52 +0000
                Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-04 20:31 -0500
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-05 01:45 +0000
                Re: Memory management and custom allocator Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-01-07 05:55 +0200
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-07 12:02 +0000
          Re: Memory management and custom allocator Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-01-01 09:08 -0500
            Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-01 10:08 -0800
              Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-01 23:34 +0000
                Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-02 02:18 -0800
                Re: Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-02 02:34 -0800
                Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-02 02:43 -0800
                Re: Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-02 03:11 -0800
                Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-02 08:17 -0800
                Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-02 11:40 +0000
                Re: Memory management and custom allocator FtM <fmassei@gmail.com> - 2012-01-02 08:27 -0800
          Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-01 23:26 +0000
            Re: Memory management and custom allocator Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-01 23:54 +0000
              Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-02 00:32 +0000
          Re: Memory management and custom allocator Kaz Kylheku <kaz@kylheku.com> - 2012-01-02 02:46 +0000
            Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-02 11:53 +0000
              Re: Memory management and custom allocator Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-02 15:18 +0000
          Re: Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-03 12:21 +0000
          Re: Memory management and custom allocator Karl Malbrain <malbrain@yahoo.com> - 2012-01-04 16:48 -0800
            Re: Memory management and custom allocator "BartC" <bc@freeuk.com> - 2012-01-05 01:36 +0000
  Re: [Slightly OT] Memory management and custom allocator "io_x" <a@b.c.invalid> - 2012-01-01 17:02 +0100
  Re: [Slightly OT] Memory management and custom allocator jgk@panix.com (Joe keane) - 2012-01-01 21:21 +0000
  Re: [Slightly OT] Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-01 21:01 -0800
  Re: [Slightly OT] Memory management and custom allocator 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-07 05:20 -0800

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