I have an ISP that sometimes throttles my bandwidth.
My physical interface to the Modem is 1Gbit, but the bandwidth to the Internet is obviously a lot less that that.
It should be 25 Mbits, but sometimes it slows down to 10 Mbits for few days.
Ive seem many tutorials of setting QoS queues specifying total bandwidth and partial bandwidth for specific traffic.

is there any way to set aside 2 Mbits for certain type of traffic (VoIP for example) , regardless of the total available bandwidth?
I want to make sure certain traffic gets at least 2 Mbits, leaving the rest for anything else (web browsing, email, chat, etc)

is there any way to set aside 2 Mbits for certain type of traffic (VoIP for example) , regardless of the total available bandwidth?
I want to make sure certain traffic gets at least 2 Mbits, leaving the rest for anything else (web browsing, email, chat, etc)

No - you might be able to build a script to check for packet loss to try to dynamically determine the total bandwidth available, but that is about the best you could do. Also each time you change the limit the queue is destroyed and rebuilt which means it doesn't exist for a second or two after the change, so if you did a calculation like that you would not want to do it too often.

is there any way to set aside 2 Mbits for certain type of traffic (VoIP for example) , regardless of the total available bandwidth?
I want to make sure certain traffic gets at least 2 Mbits, leaving the rest for anything else (web browsing, email, chat, etc)

No - you might be able to build a script to check for packet loss to try to dynamically determine the total bandwidth available, but that is about the best you could do. Also each time you change the limit the queue is destroyed and rebuilt which means it doesn't exist for a second or two after the change, so if you did a calculation like that you would not want to do it too often.

Is this a MIkrotik limitation or a QoS limitation in general?
How other vendors handle this?

Is this a MIkrotik limitation or a QoS limitation in general?
How other vendors handle this?

This is a general QoS limitation. With all products you need to enter a maximum rate and they use this to determine if the upstream provider is congested. If you are below this maximum rate and the provider is already congested, the QoS will not function properly. In general, if you need QoS, you are then forced to limit your rates to whatever the slowest speed you might ever get from your upstream provider typically is.

You can try this: viewtopic.php?f=9&t=129294

That is not a typical feature for QoS, but it looks like someone has implemented it.