User Manual

Rev 2.2-1.0.1
Mellanox Technologies
207
Note, that the same VLs may be listed multiple times in the High or Low priority arbitration
tables, and, further, it can be listed in both tables. The limit of high-priority VLArb table
(qos_<type>_high_limit) indicates the number of high-priority packets that can be transmitted
without an opportunity to send a low-priority packet. Specifically, the number of bytes that can
be sent is high_limit times 4K bytes.
A high_limit value of 255 indicates that the byte limit is unbounded.
If the 255 value is used, the low priority VLs may be starved.
A value of 0 indicates that only a single packet from the high-priority table may be sent before an
opportunity is given to the low-priority table.
Keep in mind that ports usually transmit packets of size equal to MTU. For instance, for 4KB
MTU a single packet will require 64 credits, so in order to achieve ef
fective VL arbitration for
packets of 4KB MTU, the weighting values for each VL should be multiples of 64.
Below is an example of SL2VL and VL Arbitration configuration on subnet:
qos_ca_max_vls 15
qos_ca_high_limit 6
qos_ca_vlarb_high 0:4
qos_ca_vlarb_low 0:0,1:64,2:128,3:192,4:0,5:64,6:64,7:64
qos_ca_sl2vl 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,7
qos_swe_max_vls 15
qos_swe_high_limit 6
qos_swe_vlarb_high 0:4
qos_swe_vlarb_low 0:0,1:64,2:128,3:192,4:0,5:64,6:64,7:64
qos_swe_sl2vl 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,7
In this example, there are 8 VLs configured on subnet: VL0 to VL7. VL0 is defined as a high pri-
ority VL, and it is limited to 6 x 4KB = 24KB in a single transmission burst. Such configuration
would suilt
VL that needs low latency and uses small MTU when transmitting packets. Rest of
VLs are defined as low priority VLs with different weights, while VL4 is effectively turned off.
8.6.8 Deployment Example
Figure 7 shows an example of an InfiniBand subnet that has been configured by a QoS manager
to provide different service levels for various ULPs.