Starting DRD is very similar to starting the other tools--we just have to specify our desired tool like this:

$ valgrind --tool=drd --log-file=dispatcher_drd.log --read-var-info=yes ./dispatcher_demo

After the application finishes, we examine the generated log file's contents.

==6576== drd, a thread error detector
==6576== Copyright (C) 2006-2015, and GNU GPL'd, by Bart Van Assche.
==6576== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==6576== Command: ./dispatcher_demo
==6576== Parent PID: 2838
==6576== 
==6576== Conflicting store by thread 1 at 0x05ce51b1 size 1
==6576==    at 0x403650: Worker::setRequest(AbstractRequest*) (worker.h:38)
==6576==    by 0x403253: Dispatcher::addRequest(AbstractRequest*) (dispatcher.cpp:70)
==6576==    by 0x409132: main (main.cpp:63)
==6576== Address 0x5ce51b1 is at offset 97 from 0x5ce5150. Allocation context:
==6576==    at 0x4C3150F: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_drd-amd64-linux.so)
==6576==    by 0x40308F: Dispatcher::init(int) (dispatcher.cpp:38)
==6576==    by 0x4090A0: main (main.cpp:51)
==6576== Other segment start (thread 2)
==6576==    at 0x4C3818C: pthread_mutex_unlock (in /usr/lib/valgrind/vgpreload_drd-amd64-linux.so)
==6576==    by 0x401D00: __gthread_mutex_unlock(pthread_mutex_t*) (gthr-default.h:778)
==6576==    by 0x402131: std::mutex::unlock() (mutex:153)
==6576==    by 0x403399: Dispatcher::addWorker(Worker*) (dispatcher.cpp:110)
==6576==    by 0x401DF9: Worker::run() (worker.cpp:49)
==6576==    by 0x408FA4: void std::_Mem_fn_base<void (Worker::*)(), true>::operator()<, void>(Worker*) const (in /media/sf_Projects/Cerflet/dispatcher/dispatcher_demo)
==6576==    by 0x408F38: void std::_Bind_simple<std::_Mem_fn<void (Worker::*)()> (Worker*)>::_M_invoke<0ul>(std::_Index_tuple<0ul>) (functional:1531)
==6576==    by 0x408E3F: std::_Bind_simple<std::_Mem_fn<void (Worker::*)()> (Worker*)>::operator()() (functional:1520)
==6576==    by 0x408D47: std::thread::_Impl<std::_Bind_simple<std::_Mem_fn<void (Worker::*)()> (Worker*)> >::_M_run() (thread:115)
==6576==    by 0x4F04C7F: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==6576==    by 0x4C3458B: ??? (in /usr/lib/valgrind/vgpreload_drd-amd64-linux.so)
==6576==    by 0x53EB6B9: start_thread (pthread_create.c:333)
==6576== Other segment end (thread 2)
==6576==    at 0x4C3725B: pthread_mutex_lock (in /usr/lib/valgrind/vgpreload_drd-amd64-linux.so)
==6576==    by 0x401CD1: __gthread_mutex_lock(pthread_mutex_t*) (gthr-default.h:748)
==6576==    by 0x402103: std::mutex::lock() (mutex:135)
==6576==    by 0x4023F8: std::unique_lock<std::mutex>::lock() (mutex:485)
==6576==    by 0x40219D: std::unique_lock<std::mutex>::unique_lock(std::mutex&) (mutex:415)
==6576==    by 0x401E33: Worker::run() (worker.cpp:52)
==6576==    by 0x408FA4: void std::_Mem_fn_base<void (Worker::*)(), true>::operator()<, void>(Worker*) const (in /media/sf_Projects/Cerflet/dispatcher/dispatcher_demo)
==6576==    by 0x408F38: void std::_Bind_simple<std::_Mem_fn<void (Worker::*)()> (Worker*)>::_M_invoke<0ul>(std::_Index_tuple<0ul>) (functional:1531)
==6576==    by 0x408E3F: std::_Bind_simple<std::_Mem_fn<void (Worker::*)()> (Worker*)>::operator()() (functional:1520)
==6576==    by 0x408D47: std::thread::_Impl<std::_Bind_simple<std::_Mem_fn<void (Worker::*)()> (Worker*)> >::_M_run() (thread:115)
==6576==    by 0x4F04C7F: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==6576==    by 0x4C3458B: ??? (in /usr/lib/valgrind/vgpreload_drd-amd64-linux.so) 

The preceding summary basically repeats what we saw with the Helgrind log. We see the same data race report (conflicting store), which we can safely ignore due to atomics. For this particular code at least, the use of DRD did not add anything we didn't already know from using Helgrind.

Regardless, it's always a good idea to use both tools just in case one tools spots something which the other didn't.