can someone tell me the funtion of mangle set priority which 0-7 ?
is it good for QOS for game connection, mark packet game ?
which one is good ? prerouting or postrouting ?
The “priority” and “DSCP” fields in packets are only used to deliver information about the required priority to the neighboring equipment; whether it cares about that information or ignores it is a decision of the administrator of that equipment.
Unless you live in a very specific country, your ISP doesn’t care about the priority you set in the packets you send to the internet, because in fact they are not allowed to do so if the “net neutrality” rule is in force in your country. And even if not for that - if the ISPs did respect the customers’ priority indicators, every customer would set the highest priority and the outcome would be the same like now - the ISPs would just treat all that “highest priority” traffic equally, like they do now while ignoring the priority indicators.
So what you can only affect is how your own router will prioritize the gaming packets over all the other ones as it sends them to the internet. For that, you have to have a look at Queue Tree and packet marks. The Packet Flow diagram shows that you can assign the packet mark in any of the mangle chains (prerouting/forward/postrouting), as it is actually used to choose the desired queue for the packet in the postrouting chain after the mangle table has processed the packet. And you can refer to the priority field and/or the DSCP field in the mangle rules while handling received packets to assign them a packet-mark to place them to the right queues at egress.
In the download direction (internet to you), there is no way to prioritize the packets arriving to you. You can only reduce the download bandwidth occupied by browsing etc. by queueing the delivery to the LAN hosts, but it only affects some types of traffic (all TCP traffic and some kinds of traffic using UDP), so that the packets would not get queued at the ISP as it enforces your contracted bandwidth.