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[Gcl-devel] Re: gzipped tar file on profiling

From: Camm Maguire
Subject: [Gcl-devel] Re: gzipped tar file on profiling
Date: 17 Dec 2003 17:11:44 -0500
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2

Thanks Matt!  I got it now.  I wonder if you had also saved the gbc
output -- if I recall you usually run with *notify-gbc* turned on.
Also, a (room) before and after would be of interest, as well as the
effect of (si:sgc-on nil).

On the one hand, GBC is clearly the performance culprit, and that is
very good news, as it is relatively easy to address with a better
memory layout.  What is odd about your profiling though is that
marking is taking up the lion's share of the time.  In normal GBC, its
the sweeper that costs, at least in the tests I've run so far.  And I
think I know why, it is the following extra pass that's required with
sgc marking:

  /* mark all non recent data on writable pages */
    int t,i=page(heap_end);
    struct typemanager *tm;
    char *p;
    while (--i >= 0) {
      if (WRITABLE_PAGE_P(i)
          && (t=type_map[i]) < (int) t_end)
      if ( t == t_cons) 
        for (j = tm->tm_nppage; --j >= 0; p += tm_table[t_cons].tm_size/*  
sizeof(struct cons) */) {
          object x = (object) p; 
          if (SGC_OR_M(x)) 
          if (x->d.t==t_cons) {
            x->d.m = TRUE; 
          } else
      else {
        int size=tm->tm_size;
        for (j = tm->tm_nppage; --j >= 0; p += size) {
          object x = (object) p; 
          if (SGC_OR_M(x)) continue;

I'll have to think a bit about why this was put in, but in short, sgc
need not be a performance boost if most of your memory is writable
anyway, and might (speculation) even be less efficient.  This pertains
to that note I sent earlier about the enabling of SGC in ACL2, which
you asked me to clarify, and I never did, as I don't yet have a good
answer as to a metric to determine when SGC helps and when it does
not.  One thing should be clear, and that is many heap expansions,
which require an (si::sgc-on nil)(gbc t)(si::sgc-on t), are very
expensive, as these steps require that the read-only subset of memory
be determined all over again.

To be a bit more verbose, SGC works by taking the memory set at time
of (si:sgc-on t), making it read-only, and allocating new writable
pages for use in subsequent allocation -- these are "SGC" pages,
i.e. the memory set is divided into two.  Write attempts on the old
memory make those pages writable, but they are not "SGC" pages as far
as the algorithm goes, and must be garbage collected separately.  The
idea is to enable sgc at the point where the most memory that will
essentially have no further modifications can be set aside.  Setting
aside a small amount of truly read-only memory, or a lot of memory
which has significant subsequent modifications, both suffer from

>From looking at b.out, for example, half the time is spend in
sgc_mark_phase itself, and half in its main child process,
sgc_mark_object1.  This suggests to me that the ostensibly read-only
memory which was actually modified and thus made writable (writable
non-SGC pages) is substantial.  The loop shown above is similar to
that in the sweeper which is the main cost in normal GBC, so the more
memory it has to process, the worse performance will be.

Anyway, these are just initial thoughts.  I'd like Vadim's input if
possible to construct for this case an analog of the maxima init file
he's just put together, so we can get a performance comparison with
ACL short of GBC.  This in turn will help focus our efforts on where
our out of the box algorithms and parameter settings can be improved. 

Take care, 

Matt Kaufmann <address@hidden> writes:

> Camm --
> In case email from AMD to you is problematic, I've also just now put the
> profile results on the web as a gzipped tar file:
> /u/www/users/moore/acl2/seminar/temp/camm.tar.gz
> or
> http://www.cs.utexas.edu/users/moore/acl2/seminar/temp/camm.tar.gz
> When you let me know you've received it, I'll delete this file.
> -- Matt

Camm Maguire                                            address@hidden
"The earth is but one country, and mankind its citizens."  --  Baha'u'llah

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