I believe that the "-engine classic" option is being utilized in order to make use of the deprecated "-ports" flag. However, I observed that when using such option, the maximum memory size allocated to the emulator is significantly reduced:
12:55:48 [android] Using Android SDK: C:\AndroidSDK
12:55:48 $ C:\AndroidSDK/platform-tools/adb.exe start-server
12:55:52 * daemon not running. starting it now on port 5861 *
12:55:52 * daemon started successfully *
12:55:52 $ C:\AndroidSDK/platform-tools/adb.exe start-server
12:55:52 [android] Starting Android emulator
12:55:52 $ C:\AndroidSDK/tools/emulator.exe -engine classic -ports 5794,5795 -report-console tcp:5837,max=60 -avd masterDevice -no-snapshot-load -no-snapshot-save -no-window
12:55:53 emulator: WARNING: Classic qemu does not support SMP. The hw.cpu.ncore option from your config file is ignored.
12:55:53 WARNING: init: Missing WGL extension WGL_ARB_make_current_read
12:55:53 emulator: WARNING: Requested RAM size of 3072MB is too large for your environment, and is reduced to 1728MB.
12:55:53 emulator: device fd:936
12:55:53 HAXM is working and emulator runs in fast virt mode
12:55:54 emulator: warning: opening audio input failed
12:55:58 [android] Emulator reported that the console is available on port 5,794
12:55:58 [android] Waiting for emulator to finish booting...
When using the new "-port" flag however, and otherwise starting the emulator directly on my machine. When executing command "emulator.exe -port 5566 @masterDevice" the console outputs:
Starting AVD without boot animation.
WARNING: init: Missing WGL extension WGL_ARB_make_current_read
Hax is enabled
Hax ram_size 0xc0000000
HAX is working and emulator runs in fast virt mode.
audio: Failed to create voice `goldfish_audio_in'
qemu-system-x86_64.exe: warning: opening audio input failed
console on port 5566, ADB on port 5567
Note that I have 8 emulators running in parallel on my build slave. As of now, I have all 8 running in windowed mode, with HAXM acceleration, all with 3gb of memory allocated (you can see the 3gb of memory via the settings -> memory in the emulator). All of them are running API 23 Intel x86 Atom_64 System Image.
I've tested the same command using the "-engine classic" option and the deprecated "-ports" option, and only then is my memory reduced.