LUDOC-328 tbf: add HTC strategy to NRS-TBF section

[doc/manual.git] / LustreTuning.xml

diff --git a/LustreTuning.xml b/LustreTuning.xml
index 055fe93..5b89770 100644 (file)
--- a/LustreTuning.xml
+++ b/LustreTuning.xml
@@ -1866,6 +1866,39 @@ iozone_user1 opcode={ost_read ost_write} 200, ref 0
 computes nid={192.168.1.[2-128]@tcp} 500, ref 0
 default * 10000, ref 0</screen>
          </listitem>
+         <listitem>
+           <para><emphasis role="bold">TBF realtime policies under congestion
+           </emphasis></para>
+           <para>During TBF evaluation, we find that when the sum of I/O
+           bandwidth requirements for all classes exceeds the system capacity,
+           the classes with the same rate limits get less bandwidth than if
+           preconfigured evenly. The reason for this is the heavy load on a
+           congested server will result in some missed deadlines for some
+           classes. The number of the calculated tokens may be larger than 1
+           during dequeuing. In the original implementation, all classes are
+           equally handled to simply discard exceeding tokens.</para>
+           <para>Thus, a Hard Token Compensation (HTC) strategy has been
+           implemented. A class can be configured with the HTC feature by the
+           rule it matches. This feature means that requests in this kind of
+           class queues have high real-time requirements and that the bandwidth
+           assignment must be satisfied as good as possible. When deadline
+           misses happen, the class keeps the deadline unchanged and the time
+           residue(the remainder of elapsed time divided by 1/r) is compensated
+           to the next round. This ensures that the next idle I/O thread will
+           always select this class to serve until all accumulated exceeding
+           tokens are handled or there are no pending requests in the class
+           queue.</para>
+           <para>Command:</para>
+           <para>A new command format is added to enable the realtime feature
+           for a rule:</para>
+           <screen>lctl set_param x.x.x.nrs_tbf_rule=\
+"start <replaceable>rule_name</replaceable> <replaceable>arguments</replaceable>... realtime=1</screen>
+           <para>Example:</para>
+           <screen>$ lctl set_param ost.OSS.ost_io.nrs_tbf_rule=
+"start realjob jobid={dd.0} rate=100 realtime=1</screen>
+           <para>This example rule means the RPC requests whose JobID is dd.0
+           will be processed at the rate of 100req/sec in realtime.</para>
+         </listitem>
        </itemizedlist>
       </section>
     </section>