Extremely large amount of data in jos_session

I noticed yesterday that my joomla/virtuemart site was becoming very slow.

I contacted the hosting provider and they told me about queries to jos_session. I looked in phpmyadmin at the table jos_session a开发者_StackOverflownd noticed that the data field for each session id had an insane amount of lines. A total of 157.080 characters to be exact.

At the time when this came up I was uploading products to virtuemart with csvimproved. Everything had been smooth and I had around 3000 product on the site but now when I have around 10.000 products everything is very slow.

I don't know where this data in jos_session is coming from and I'm not certain that it's related to these products.

Do you guys have any idea what is happening or how I could find out?

Thanks

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If you don't need the session in a public page, you can use this in index.php:

$mainframe =& JFactory::getApplication('site',array('session'=>false));