PCQ queues and 1500-bytes ping

RouterOS General
1

Hello. I’ve network scheme:
Subscribers - <…> Cisco3550 - Mikrotik (192.168.1.2) - (192.168.1.3) Cisco3825

and PCQ queues:

 0   name="Download512" parent=ether1_cisco3550 packet-mark=downpack512 limit-at=0  
     queue=pcq-download512 priority=8 max-limit=0 burst-limit=0 burst-threshold=0  
     burst-time=0s  
 
 1   name="Upload512" parent=ether2_cisco3825 packet-mark=uppack512 limit-at=0  
     queue=pcq-upload512 priority=8 max-limit=0 burst-limit=0 burst-threshold=0 burst-time=0s

Queues work fine but I’ve really strange fact: if my subscribers ping interface 192.168.1.3 (Cisco 3825) with 1500-bytes packets, ping time is about 60-63 ms. If they ping 192.168.1.3 with packets 1450-1470 bytes, ping becomes ideal: 8-10 ms.

When I use parent=global-out instead of defined interface ether2_cisco3825 in the queue Upload512, 1500-bytes ping time is almost half-smaller: 25-29 ms.

After disabling PCQ queues 1500-bytes ping time is 8-12 ms. Could you please explain me this fact or help me with this problem?

Thanks for your ideas and sorry for my Eng :slight_smile: