[squid-users] squid queue overload. request rejected

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Hello,
 
From last week I am getting messages in my cache.log "squid queue overload. 
request rejected" and most of my users unable to browse the webpages. I have 
configured squid with external helper ACL and using squid stable version 
squid2.7 stable 9. Any idea about this error.
 
 
Regards,
Sarfraz