README: Document the latency measurement infrastructure

git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@7903 d57e44dd-8a1f-0410-8b47-8ef2f437770f

scst/README

@@ -448,6 +448,42 @@ following entries:
    CPUs serving soft IRQs and in some cases to improve performance by
    more evenly spreading load over available CPUs.
 
+ - measure_latency - whether or not to enable latency measurements.
+   Enabling latency measurements has a small impact on performance but
+   makes detailed information available about how much time is needed
+   to process SCSI commands. Here is an example of the data produced
+   by this infrastructure:
+    
+     $ echo 1 >/sys/kernel/scst_tgt/measure_latency
+     $ sleep 10 # Wait until an initiator has submitted multiple I/O requests
+     $ cat /sys/kernel/scst_tgt/targets/ib_srpt/fe80\:0000\:0000\:0000\:0002\:c903\:00fa\:b7f2/sessions/fe80\:0000\:0000\:0000\:0002\:c903\:00a3\:4272/latency/r512
+     state             count  min    max   avg stddev
+     PARSE               219  1.3   26.6   2.2    2.5 us
+     PREPARE_SPACE       219  0.9   10.3   1.1    0.6 us
+     RDY_TO_XFER         219  0.7    1.7   0.7    0.2 us
+     TGT_PRE_EXEC        219  0.7   11.0   0.8    0.9 us
+     EXEC_CHECK_SN       219  0.7    1.7   0.8    0.2 us
+     PRE_DEV_DONE        219 11.3 3445.7  39.6  276.4 us
+     DEV_DONE            219  0.7   11.0   0.9    0.7 us
+     PRE_XMIT_RESP1      219  1.2   58.4   1.6    3.8 us
+     CSW2                219  0.7    1.6   0.8    0.1 us
+     PRE_XMIT_RESP2      219  0.7    1.5   0.7    0.1 us
+     XMIT_RESP           219  0.7    1.5   0.7    0.1 us
+     INIT_WAIT           219  1.0   57.3   2.1    4.4 us
+     INIT                219  0.9   27.4   1.6    2.4 us
+     CSW1                219 15.0 3856.1  74.2  264.8 us
+     EXEC_CHECK_BLOCKING 219  1.3   10.8   1.7    0.9 us
+     LOCAL_EXEC          219  0.7    1.8   0.7    0.1 us
+     REAL_EXEC           219  0.6    1.5   0.7    0.1 us
+     EXEC_WAIT           219 40.6 1021.7  54.4   68.7 us
+     XMIT_WAIT           219  6.4 1682.0  50.6  228.1 us
+     total               219    -      - 236.9 2012.1 us
+
+   PRE_DEV_DONE refers to internal checks done after execution of a command
+   finished. CSW1 is the context switch that happens after the transport
+   driver received a command and before processing of a command starts.
+   EXEC_WAIT is the time spent in the device handler .exec() method.
+
  - sgv - this is a root subdirectory for all SCST SGV caches
 
  - targets - this is a root subdirectory for all SCST targets