Subject: Re: kern/1718: MAXBSIZE too low.
To: None <cgd@NetBSD.ORG>
From: Martin Cracauer <cracauer@wavehh.hanse.de>
List: netbsd-bugs
Date: 11/03/1995 17:27:35
In netbsd-bugs you write:

>	MAXBSIZE was #defined to be MAXPHYS in 4.4BSD-Lite (and Lite2).
>	It was changed to be 16k in NetBSD.  (If my memory serves me,
>	the reason given at the time was to save kernel page table space
>	on the i386.

>	This drastically reduces the effectiveness of clustering (esp.
>	write clustering), because the number of I/Os which can be combined
>	is reduced.  (Coalsced I/Os must still fit into a buffer with
>	maximum size of MAXBSIZE.)

Wait a moment, please.

Setting MAXBSIZE=MAXPHYS=64k on my i386/sup19950927 panics at boot
time. 

Setting MAXBSIZE to more that 16 K and up to 57K works and gives
the following bonnie results. The machine is P90, triton, NCR 810,
barracouda 2 GB.
              -------Sequential Output-------- ---Sequential Input----Random--
              -Per Char- --Block--- -Rewrite-- -Per Char- --Block-----Seeks---
MAXBSIZE=16K (default)
          140  1427 99.5  2806 19.6   793 10.2  1625 99.5  4573 32.9  83.6  7.0
          140  1432 99.4  2801 18.4   793  9.9  1636 99.7  4589 32.1  84.0  6.8
MAXBSIZE=32K
          140  1453 99.4  4276 26.8  1259 13.2  1659 99.7  4593 27.2  84.1  6.8
MAXBSIZE=48K
          140  1466 99.5  4215 25.6  1746 17.1  1673 99.7  4592 25.2  83.0  6.7
MAXBSIZE=56K
          140  1463 99.5  4209 25.5  1504 14.5  1679 99.5  4592 24.7  84.5  6.9
MAXBSIZE=64K
  doesn't boot

FreeBSD-951005-SNAP (no flames please :-)
          140  1383 98.8  4557 42.6  1642 18.3  1646 98.3  4595 27.7  92.5  7.9
[ note that FreeBSD needs more CPU cycles ]

Some backtrace points of the panic
_pmap_enter
_vm_fault
...
_kmem_alloc
...
_vm_fork
...
_main


Maybe this is related:
./arch/i386/isa/fdreg.h:
#define FDC_MAXIOSIZE   NBPG    /* XXX should be MAXBSIZE */

The comment doesn't make sense. NBPG is 4k, not 16k. Setting 
#define FDC_MAXIOSIZE MAXBSIZE 
doesn't make a working kernel (doesn't find boot disk anymore).

Martin
-- 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Martin Cracauer <cracauer@wavehh.hanse.de>  -  Fax +49 40 522 85 36
 BSD User Group Hamburg, Germany - See http://{net,free}bsd.www.org
 Copyright 1995. Redistribution via Microsoft Network is prohibited