use strict;
use warnings;

use constant CACHESIZE => 2**24; ## Default size of cache memory (2**24=>16_777_216)
use constant PAGESIZE  => 2048;		

our %RCache = (); keys( %RCache ) = 1020;

$dbenv{DB_RMaxkeys} = CACHESIZE / PAGESIZE;
. . .
if ( scalar keys %RCache > $dbenv{DB_RMaxkeys} ) { &CheckMemoryUsage('WR'); }
. . .

if ( scalar keys %RCache > $dbenv{DB_RMaxkeys} ) { &CheckMemoryUsage('RN'); }
. . .

if ( scalar keys %RCache > $dbenv{DB_RMaxkeys} ) { &CheckMemoryUsage('RP'); }
. . .

# my $ret = &GetSubBuffer(\$db,$subtreeptr,\$buffer, $log )
sub GetSubBuffer
{	my ( $db, $ptr, $buffer, $log ) = @_; my $size = $dbenv{DB_InternalPageSize};
	if ( ( $CACHE == 1 )&&( exists $RCache{$ptr} ) )
	{	$$buffer = $RCache{$ptr}; }
	else
	{	if ( $ptr < $size ) { die "  GetSubBuffer: $log subtreeptr $ptr <= 0\n";  }
		$ret = sysseek( $$db{btree}, $ptr, 0);		# move to subtree location in file
		if ( ! defined $ret ) { die "  GetSubBuffer: $log sysseek failed:|$ptr| $!\n"; }
		$ret = sysread( $$db{btree},my $tmpbuf, $size );
		if ( $ret != $size ) { die "  GetSubBuffer: $log sysread failed: $!\n"; }
		my $reclen = unpack("N", substr($tmpbuf,0,4));

		if ( ! defined $$buffer ) { die "$log buffer not defined!"; }
		substr($$buffer,0,$reclen+4,$tmpbuf); #*#
		if ( $CACHE == 1 )
		{	$RCache{$ptr} = $$buffer; }
	}
}
sub CheckMemoryUsage
{	our %RCache;
	use Devel::Size qw(total_size);
	my $log = shift;
	my $stime = gettimeofday; my $keys = scalar keys %RCache;
	print $DLOG "MEM_CK-$log: Enter:  Keys:$keys   \%RCache Size: ",total_size(\%RCache),"\n";
	my ( $vmem, $rmem ) = &Display_Mem_Usage($$,$NAME,0);

	my $rkeys = scalar keys %RCache;

##</code><code>##

##   1. Failing code:

	%RCache = (); keys( %RCache ) = 1020;	## THIS DOESN'T WORK AFTER 1ST PASS

##</code><code>##

##   2. This works:
	if ( $rkeys > $dbenv{DB_RMaxkeys} )
	{	my $killkeys = int($rkeys/2);
		foreach my $key ( keys %RCache )
		{	$killkeys--; if ( $killkeys < 0 ) { last; }
			delete $RCache{$key};
		}
	}

##</code><code>##

	my $etime = sprintf("%.4f",gettimeofday - $stime);
	$keys = scalar keys %RCache;
	print $DLOG "           Exit:   Keys:$keys   \%RCache Size: ",total_size(\%RCache),"   Time:$etime\n";
}


##</code><code>##

############################# Tue Jan  3 10:49:32 2012
Start. . . 
## Start: VSZ-6928_KB-0 RSS-3988_KB-0	BLOCK: 2048  Tue Jan  3 10:49:32 2012
MEM_CK-WR: Enter:  Keys:8193   %RCache Size: 16027140
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0304
MEM_CK-WR: Enter:  Keys:8193   %RCache Size: 70629951
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0375
MEM_CK-WR: Enter:  Keys:8193   %RCache Size: 104741100
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0399
MEM_CK-WR: Enter:  Keys:8193   %RCache Size: 105994568
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0406
MEM_CK-WR: Enter:  Keys:8193   %RCache Size: 111110214
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0414
MEM_CK-RN: Enter:  Keys:8193   %RCache Size: 112428340
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0480
MEM_CK-RN: Enter:  Keys:8193   %RCache Size: 17416496
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0492
MEM_CK-RN: Enter:  Keys:8193   %RCache Size: 17416495
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0526
MEM_CK-RP: Enter:  Keys:8264   %RCache Size: 253096813
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0644
MEM_CK-RP: Enter:  Keys:8220   %RCache Size: 400929483
           Exit:   Keys:0   %RCache Size: 65592   Time:0.0782
## End: VSZ-465472_KB-0 RSS-462424_KB-0  Diff: 458544|458436_KB-0
####### Devel::Size #########
%RCache: 374,120,916  No of keys: 7481

##</code><code>##

############################# Tue Jan  3 10:01:59 2012
Start. . . 
## Start: VSZ-6928_KB-0 RSS-3988_KB-0	BLOCK: 2048  Tue Jan  3 10:01:59 2012
MEM_CK-WR: Enter:  Keys:7325   %RCache Size: 14293576
           Exit:   Keys:3663   %RCache Size: 7165898   Time:0.0285
MEM_CK-WR: Enter:  Keys:7325   %RCache Size: 16593819
           Exit:   Keys:3663   %RCache Size: 7170630   Time:0.0300
. . .
MEM_CK-WR: Enter:  Keys:7392   %RCache Size: 31464649
           Exit:   Keys:3696   %RCache Size: 7251138   Time:0.0353
MEM_CK-WR: Enter:  Keys:7326   %RCache Size: 31293329
           Exit:   Keys:3663   %RCache Size: 7189706   Time:0.0348
MEM_CK-RN: Enter:  Keys:7325   %RCache Size: 30792296
           Exit:   Keys:3663   %RCache Size: 7189758   Time:0.0339
MEM_CK-RN: Enter:  Keys:7325   %RCache Size: 14945032
           Exit:   Keys:3663   %RCache Size: 7189758   Time:0.0381
. . .
MEM_CK-RN: Enter:  Keys:7325   %RCache Size: 14945069
           Exit:   Keys:3663   %RCache Size: 7189758   Time:0.0490
MEM_CK-RP: Enter:  Keys:7361   %RCache Size: 26095864
           Exit:   Keys:3681   %RCache Size: 7280837   Time:0.0484
. . .
MEM_CK-RP: Enter:  Keys:7361   %RCache Size: 31337851
           Exit:   Keys:3681   %RCache Size: 7291441   Time:0.0477
## End: VSZ-127068_KB-0 RSS-124144_KB-0  Diff: 120140|120156_KB-0
####### Devel::Size #########
%RCache: 21,012,262  No of keys: 5763