BrowserUk:

Since I can only ++ a post once, you'll have to settle for a few more virtuals: ++ ++ ++

...roboticus

When your only tool is a hammer, all problems look like your thumb.

I actually originally started by only loading data from the DB in smaller chunks - the way I'm using that DB allows me to split the whole table into about 5000 smaller ones. So instead of doing one SELECT and creating one huge hash (which always felt a bit wrong to me), I did 5000 smaller SELECTs and worked through those in sequence.

Performance of this however was terrible - roughly speaking about 20 seconds for loading that 1 big SELECT from DB vs 220 seconds for 5000 smaller SELECTs - all via one DB handle. And this was on SSD.

As for the in-memory DB, this is the first time I've heard about it and admittedly it looks very promising. Thanks for the hint - let me give that a try...

@BrowserUK, what exactly do you mean by this?

<quote>Instead, share the db handle and create statement handles for your queries. </quote>

I'm using DBI and that doesn't seem to be very thread friendly. Also SQLite DBD doesn't mention threading anywhere in the documentation.

When I try to pass the db handle to my thread as a parameter, I get this:

$g_dbh = DBI->connect("dbi:SQLite:dbname=:memory:");
threads->create(\&my_thread, $g_dbh);

Thread 1 terminated abnormally: DBD::SQLite::db prepare failed: handle
+ 2 is owned by thread 7f7f64003200 not current thread 7f7f6455fc00 (h
+andles can't be shared between threads and your driver may need a CLO
+NE method added)
[download]

If I try to share the db handle, I get:

our $g_dbh :shared;
$g_dbh = DBI->connect("dbi:SQLite:dbname=:memory:");

Invalid value for shared scalar
[download]

Okay. I found an obscure detail(Section entitled "In-memory Databases And Shared Cache" ) in the sqlite documentation that allows me to work around DBI's brokeness.

This demonstrates it:

#! perl -slw
use strict;
use threads;
use threads::shared;
use threads::Q;
use Time::HiRes qw[ time ];
use DBI;

use constant {
    CONNECT=> 'dbi:SQLite:dbname=file:memdb?mode=memory&cache=shared',
    CREATE => 'create table if not exists DB ( ID integer(8),'
        . join(',', map "F$_ text(15)", 1..9) . ')',
    INSERT => 'insert into DB ( ID , '
        . join( ',', map "F$_", 1..9 ) . ') values (' . '?,' x 9 . '?)
+',
    INDEX  => 'create index if not exists I1 on DB ( ID )',
    QUERY  => 'select * from DB where ID = ?',
};

sub thread {
    my $tid = threads->tid;
    my( $Q ) = @_;
    $Q->dq; ## Wait for DB

    my $dbh = DBI->connect( CONNECT, '', '' ) or die DBI::errstr;
    my $sth = $dbh->prepare( QUERY ) or die DBI->errstr;
    while( my $id = $Q->dq ) {
        $sth->execute( $id ) or die DBI::errstr;
        my $r = $sth->fetch or warn( "No data for $id" ) and next;
        ## do something with record.
        printf "[$tid] %5u %s %s %s %s %s %s %s %s %s\n", @{ $r };
    }
    $sth->finish;
    $dbh->disconnect;
}

my @chars = ( 'a'..'z' );
sub dummy {
     my $n = shift;
     join '', @chars[ map int( rand @chars ), 1 .. $n ];
}

our $T //= 4;
our $N //= 100;

my $dbh = DBI->connect( CONNECT, '', '', { AutoCommit =>0 } ) or die D
+BI::errstr;
$dbh->do( 'PRAGMA synchronous = off' );
$dbh->do( 'PRAGMA cache_size = 800000' );
$dbh->do( CREATE ) or die DBI::errstr;

my $ins = $dbh->prepare( INSERT ) or die DBI->errstr;
for my $n ( 1 .. $N ) {
    my @fields = ( $n, map dummy( 15 ), 1 .. 9 );
    $ins->execute( @fields )or die $ins->errstr;
    $n %100 or $dbh->commit
}
$ins->finish;
$dbh->commit;
#$dbh->do( INDEX  ) or die DBI::errstr;

my $sth = $dbh->prepare( QUERY ) or die DBI->errstr;
for my $id ( 1 .. 5 ) {
    $sth->execute( $id ) or die DBI::errstr;
    my $r = $sth->fetch() or warn( "No data for $id" ) and next;
    ## do something with record.
    printf "[main] %5u %s %s %s %s %s %s %s %s %s\n", @{ $r };
}
$sth->finish;

print $dbh->selectrow_array( 'SELECT count(*) from DB' );

my $Q = threads::Q->new( 10 );
my @threads = map threads->create( \&thread, $Q, shared_clone( $dbh ) 
+), 1 .. $T;
$Q->nq( ('Wakey wakey') x $T );
$Q->nq( $_ ) for 1 .. 20; ## $N;
$Q->nq( (undef) x $T );
$_->join for @threads;
$dbh->disconnect;

unlink 'file'; ## without this, the memory db will persist in a appare
+ntly empty file????
## That might be turned into an advantage????
[download]

There is however some weirdness associated with this. (See the comments on the last two lines above).

Basically, the trick to making multiple DBI handles (in different threads) refer to the same in memory database is using a dbname of the form:

file:memdb?mode=memory&cache=shared
[download]

The weirdness is that although this is a "memory db", a file (in this example called 'file' in the local directory) is created. It will always be 0 bytes and running wc on it confirms it has no content.

But, run the above program a second time and it will add to the number of records in the table. IT WILL DOUBLE IN SIZE!

But if you delete that empty file, the records from the first run disappear!

Not sure what to do about that other than just delete the file.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

Is file:memdb making use of an alternate data stream? That would could explain the behavior that you are seeing. It would be interesting to explore the file named file to see if this was the case.

Update: Some other informational links (no assurance of quality on these):

• http://www.bleepingcomputer.com/tutorials/windows-alternate-data-streams/
• Streams utility from MS
• http://www.irongeek.com/i.php?page=security/altds

--MidLifeXis

So I finally got some time to play with this and I still can't wrap my head around it.

If I understand it correctly, the trick is in the fact that you:

1. load the whole DB as a memdb with shared cache
2. then use the

   shared_clone( $dbh )
   [download]

   to get this memory where the DB is loaded shared with all the threads
3. and then inside the thread create a new DB handler, but thanks to the fact you're using the same memdb with shared cache, it reuses the same memory as the DB handler in the main thread

Is that even remotely correct? :-) And if so:

1. Why do the shared_clone( $dbh )? Isn't the shared memdb enough for the threads to have access to the same memory? I've been trying to somehow see the difference between the two scenarios (with and without passing the shared_clone( $dbh )) and I'm probably not measuring this right, but the memory usage (vm_stat) seems to be similar in both cases.
2. What's the purpose of the

   $Q->nq( ('Wakey wakey') x $T );
   ...
   $Q->dq; ## Wait for DB
   [download]

   ? Why pass the string to the thread and then just dequeue it? And what's the relation to waiting for DB?

Wow, this is quite something. I'm trying to absorb all that and I have probably a very stupid question at the beginning - what is threads::Q? I can't find that on CPAN or anywhere else.

Sorry. It looks like we'll have to try and locate some alternative to (DBI) using sqlite from Perl.

DBI is broken beyond understanding and the documentation is so crap that working out how to fix it is beyond my abilities.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.