[Varnish] #1788: ObjIter has terrible performance profile when busyobj != NULL

[Varnish]

#1788: ObjIter has terrible performance profile when busyobj != NULL
-------------------+--------------------
 Reporter:  tnt    |       Type:  defect
   Status:  new    |   Priority:  normal
Milestone:         |  Component:  build
  Version:  4.0.3  |   Severity:  normal
 Keywords:         |
-------------------+--------------------
 The loop of ObjIter when there is a busyobj (i.e. dl from the backend
 isn't over), will do a linear scan of all the 'struct storage' for every
 chunk served to the client. This mean a N^2 complexity which is _really_
 bad.

 This causes issues when the file is largish (i.e hundreds of Mbytes), the
 download time is several order of magnitude larger through varnish than it
 is hitting the backend directly.

 Especially when the client connection is about the same speed as the
 backend connection (and so busyobj is != NULL for a while)

 I tested this on 4.1-rp1 and 4.0.3.

 To reproduce, use this .vtc

 {{{
 vcl 4.0;

 backend be {
         .host = "127.0.0.1";
         .port = "5000";
 }

 sub vcl_recv {
         return (pass);
 }

 sub vcl_backend_response {
         set beresp.do_stream = true;
 }
 }}}

 and download like a 1G file through varnish. In my case it takes 2m30s to
 download (from 127.0.0.1) and my cpu is 100% the whole time. While hitting
 the backend directly it takes 3s.

-- 
Ticket URL: <https://www.varnish-cache.org/trac/ticket/1788>
Varnish <https://varnish-cache.org/>
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