squid fails with a TCP_SWAPFAIL_MISS when handling 'n' concurrent requests for the same object

[Saurabh Sheth <saurabh.sheth@xxxxxxxxxx>]

Squid (versions: 3.1 and 2.6) has a object in its cache and responds to 
individual requests to this object just fine (TCP_HIT:NONE). From the 
access.log ->

10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 41136 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 24752 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 28848 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 41136 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 24752 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 45232 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 28848 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 49328 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 49328 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 32944 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 37040 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:41:55 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 37040 TCP_HIT:NONE


However, when I make a huge number of concurrent requests for the same 
object, squid fails to load the object from the disk fast enough and 
gives a TCP_SWAPFAIL_MISS ->


10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 53424 TCP_HIT:NONE
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 37031 TCP_SWAPFAIL_MISS:DIRECT
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 28839 TCP_MISS:NONE
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 32935 TCP_MISS:NONE



All subsequent requests hit the origin server directly causing huge load 
on the origin server (TCP_MISS:NONE) ->

10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 32935 TCP_MISS:NONE
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 28839 TCP_MISS:NONE
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 37031 TCP_MISS:NONE
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 37031 TCP_MISS:NONE
10.192.x.x - - [11/Sep/2012:15:42:23 -0700] "GET 
http://originserver/data/object HTTP/1.1" 200 32935 TCP_MISS:NONE

This is undesirable in the production setup, since such huge number of 
requests hitting the origin server directly have the result of a DOS 
attack on the origin server. This has brought down our origin server 
more than once now.


I am looking for any help or pointers on how can I deal with such a huge 
number of concurrent requests to squid for the same object effectively, 
any help is highly appreciated. I am already considering the option of 
rate limiting using iptables, however if there is a effective way to 
deal with this in the squid configuration itself; I would love to 
understand.