Php-fpm: why do I have a process memory usage higher than memory_limit

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.

pm.max_children means you can only have that many php processes running at once. If they are all taking a long time, then you are tied up, and other incoming connections must wait. To resolve this, increase the size of the php-fpm pool to the maximum you can get away with given your resource constraints.