Re: [HACKERS] BLCKSZ fun facts - Mailing list pgsql-patches

I have implemented your ideas for checking BLCKSZ >= 1024, and having
initdb adjust shared buffers checks based on BLCKSZ.

Patch attached and applied.

---------------------------------------------------------------------------

Peter Eisentraut wrote:
> The smallest BLCKSZ that you can compile is 256.  But ...
>
> The smallest BLCKSZ that actually works is 1024, because of this code in
> guc.c:
>
>     case GUC_UNIT_BLOCKS:
>         val /= (BLCKSZ / 1024);
>
> Maybe it's worth adding an #error here to prevent smaller sizes being
> used?
>
> The smallest BLCKSZ that passes the regression tests is 4096.  With
> smaller settings your get half a dozen ordering differences, which
> seems OK.
>
> The shared memory configuration code in initdb doesn't know about
> BLCKSZ, so with smaller sizes you get less shared buffers.  Maybe that
> is worth fixing sometime.
>
> Aside from that my pgbench testing clearly shows that block sizes larger
> than 2048 become progressively slower.  Go figure.
>
> --
> Peter Eisentraut
> http://developer.postgresql.org/~petere/
>
> ---------------------------(end of broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faq

--
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: src/bin/initdb/initdb.c
===================================================================
RCS file: /cvsroot/pgsql/src/bin/initdb/initdb.c,v
retrieving revision 1.133
diff -c -c -r1.133 initdb.c
*** src/bin/initdb/initdb.c    16 Feb 2007 02:10:07 -0000    1.133
--- src/bin/initdb/initdb.c    20 Feb 2007 23:46:19 -0000
***************
*** 1208,1214 ****

      for (i = 0; i < bufslen; i++)
      {
!         test_buffs = trial_bufs[i];
          if (test_buffs <= ok_buffers)
          {
              test_buffs = ok_buffers;
--- 1208,1215 ----

      for (i = 0; i < bufslen; i++)
      {
!         /* Use same amount of memory, independent of BLCKSZ */
!         test_buffs = (trial_bufs[i] * 8192) / BLCKSZ;
          if (test_buffs <= ok_buffers)
          {
              test_buffs = ok_buffers;
Index: src/include/pg_config_manual.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/pg_config_manual.h,v
retrieving revision 1.24
diff -c -c -r1.24 pg_config_manual.h
*** src/include/pg_config_manual.h    6 Feb 2007 09:16:08 -0000    1.24
--- src/include/pg_config_manual.h    20 Feb 2007 23:46:19 -0000
***************
*** 25,30 ****
--- 25,34 ----
   */
  #define BLCKSZ    8192

+ #if BLCKSZ < 1024
+ #error BLCKSZ must be >= 1024
+ #endif
+
  /*
   * RELSEG_SIZE is the maximum number of blocks allowed in one disk
   * file.  Thus, the maximum size of a single file is RELSEG_SIZE *