In version 2.2 this works: when Http client samplers and Cookie Manager with "clear cookies each iteration" enabled are put into thread group, for 20 thread running concurrently in that group there are 20 different HTTP sessions initialized for every iteration, dropped at the end of this iteration. In 2.3.1 for the same script only ONE http session is created for all 20 threads and it's not dropped on every iteration.
Also works in 2.3 (or maybe that's what you meant?). The collection used to store the permanent cookies was being shared between threads, instead of copied - oops! Fixed in SVN r602290. Thanks for the report.
Woow, that was quick :) Thank you.
If you want to try the fix, it's in the nightly r602302. Let us know if there are any problems.
*** Bug 44043 has been marked as a duplicate of this bug. ***
*** Bug 44228 has been marked as a duplicate of this bug. ***
See: http://markmail.org/message/bis6om3wnpqtqin5 for additional work-rounds
*** Bug 44763 has been marked as a duplicate of this bug. ***
*** Bug 44814 has been marked as a duplicate of this bug. ***
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/2053