[Varnish] #897: sess_mem "leak" on hyper-threaded cpu

[Varnish]

#897: sess_mem "leak" on hyper-threaded cpu
-------------------------------------------------+--------------------------
 Reporter:  askalski                             |        Type:  defect
   Status:  new                                  |    Priority:  normal
Milestone:                                       |   Component:  build 
  Version:  trunk                                |    Severity:  major 
 Keywords:  sess_mem leak n_sess race condition  |  
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Comment(by askalski):

 I wrote a short program to test the increment/decrement race condition,
 and ran it on a dual socket quad core machine with HT.  The program runs
 through a series of 2-thread trials, with each trial setting thread CPU
 affinities to a different pair (ex: thread A runs on cpu0, thread B runs
 on cpu1.)

 {{{
         for (i = arg->iterations; i > 0; --i) {
                 ++counter;
                 --counter;
         }
 }}}

 Cpu0 and cpu8 share the same physical id and core id.

 {{{
 $ ./racetest
 cpu0 vs cpu0 (1000000000 iterations)... counter drift = 2
 cpu0 vs cpu1 (1000000000 iterations)... counter drift = 318
 cpu0 vs cpu2 (1000000000 iterations)... counter drift = 709
 cpu0 vs cpu3 (1000000000 iterations)... counter drift = 313
 cpu0 vs cpu4 (1000000000 iterations)... counter drift = 690
 cpu0 vs cpu5 (1000000000 iterations)... counter drift = 336
 cpu0 vs cpu6 (1000000000 iterations)... counter drift = 578
 cpu0 vs cpu7 (1000000000 iterations)... counter drift = 359
 cpu0 vs cpu8 (1000000000 iterations)... counter drift = 13720798
 cpu0 vs cpu9 (1000000000 iterations)... counter drift = 325
 cpu0 vs cpu10 (1000000000 iterations)... counter drift = 581
 cpu0 vs cpu11 (1000000000 iterations)... counter drift = 361
 cpu0 vs cpu12 (1000000000 iterations)... counter drift = 685
 cpu0 vs cpu13 (1000000000 iterations)... counter drift = 316
 cpu0 vs cpu14 (1000000000 iterations)... counter drift = 637
 cpu0 vs cpu15 (1000000000 iterations)... counter drift = 337
 }}}

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Ticket URL: <http://www.varnish-cache.org/trac/ticket/897#comment:1>
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