Linux – How to change default of max open files per process

You have a server with 256M, but you can't use all of that -- remember there's some OS overhead. Add to that with the fact you're over committing as other folks have mentioned and you'll definitely thrash here. 256M is only enough for a small DB, 20 connections is a lot with what you've got configured.

1) reduce your max connections to 4 (you're using 3 out of 20)

2) optimize your query cache better; 8M is really large, and 64M total is a lot based on your hits/prunes; try a 4/32 combo and see how it goes. Really I think a 2/24 combo would work for you.

3) you have no sorts requiring temp tables, why is that max_heap_table_size verb in there? Comment that out, use the defaults

4) do you actually have 128 tables? Try cutting that table_cache in half to 64 or 48

5) reduce thread_cache_size to 4

6) optimize those tables to reduce fragmenting

Those are some things to start with. It looks like you threw a bunch of numbers in a config without any actual profiling to know what you needed and have created a mess; if all else fails go back to the defaults and get rid of your custom settings and start over using some performance tuning guides you can find on Google. Get the output of SHOW VARIABLES and SHOW STATUS, find any one of a bajillion tuning guides and plug in your actual, real numbers into their equations and that'll tell you the exact-ish numbers you need to put in your config file.

Background:

I just grabbed the current version of Piwik -- In version 1.5, line 969 appears as:

public function addRowsFromSerializedArray( $stringSerialized )
{
    $serialized = unserialize($stringSerialized);
    if($serialized === false)
    {
        throw new Exception("The unserialization has failed!");
    }
    $this->addRowsFromArray($serialized);
}

and is specifically $serialized = unserialize($stringSerialized);. The call to unserialize can be incredibly memory intensive. There is an excellent post here.

As you've noted, this clearly isn't a bug in your script and is a valid Out of Memory.

Suggestion: In the config file, as noted above in my comment:

/.../piwik/config/global.ini.php

I think you may need to increase one of these limits:

# during archiving, Piwik will limit the number of results recorded, for performance reasons
# maximum number of rows for any of the Referers tables (keywords, search engines, campaigns, etc.)
# this limit will also be applied to the Custom Variables names and values reports
datatable_archiving_maximum_rows_referers = 1000
# maximum number of rows for any of the Referers subtable (search engines by keyword, keyword by campaign, etc.)
datatable_archiving_maximum_rows_subtable_referers = 50

# maximum number of rows for any of the Actions tables (pages, downloads, outlinks)
datatable_archiving_maximum_rows_actions = 500
# maximum number of rows for pages in categories (sub pages, when clicking on the + for a page category)
# note: should not exceed the display limit in Piwik_Actions_Controller::ACTIONS_REPORT_ROWS_DISPLAY
#       because each subdirectory doesn't have paging at the bottom, so all data should be displayed if possible.
datatable_archiving_maximum_rows_subtable_actions = 100

# maximum number of rows for other tables (Providers, User settings configurations)
datatable_archiving_maximum_rows_standard = 500

where I changed the semi-colons to # signs just to make sf's autocolor readable.

You might also try adding:

CMD_TO_CHECK_SETTINGS="$PHP_BIN -i > /tmp/piwik-php-env.out"
$CMD_TO_CHECK_SETTINGS

to archive.sh to determine if there are other settings afoot which override the php.ini file.