[DOC][FAQ] add faq for how to deal with slow task
This closes #3514
diff --git a/docs/faq.md b/docs/faq.md
index 9ba7082..16cdfa5 100644
--- a/docs/faq.md
+++ b/docs/faq.md
@@ -29,6 +29,7 @@
* [Why all executors are showing success in Spark UI even after Dataload command failed at Driver side?](#why-all-executors-are-showing-success-in-spark-ui-even-after-dataload-command-failed-at-driver-side)
* [Why different time zone result for select query output when query SDK writer output?](#why-different-time-zone-result-for-select-query-output-when-query-sdk-writer-output)
* [How to check LRU cache memory footprint?](#how-to-check-lru-cache-memory-footprint)
+* [How to deal with the trailing task in query?](#How-to-deal-with-the-trailing-task-in-query)
# TroubleShooting
@@ -227,6 +228,29 @@
**Note:** If `Removed entry from InMemory LRU cache` are frequently observed in logs, you may have to increase the configured LRU size.
To observe the LRU cache from heap dump, check the heap used by CarbonLRUCache class.
+
+## How to deal with the trailing task in query?
+
+When tuning query performance, user may found that a few tasks slow down the overall query progress. To improve performance in such case, user can set spark.locality.wait and spark.speculation=true to enable speculation in spark, which will launch multiple task and get the result the one of the task which is finished first. Besides, user can also consider following configurations to further improve performance in this case.
+
+**Example:**
+
+```
+spark.locality.wait = 500
+spark.speculation = true
+spark.speculation.quantile = 0.75
+spark.speculation.multiplier = 5
+spark.blacklist.enabled = false
+```
+
+**Note:**
+
+spark.locality control data locality the value of 500 is used to shorten the waiting time of spark.
+
+spark.speculation is a group of configuration, that can monitor trailing tasks and start new tasks when conditions are met.
+
+spark.blacklist.enabled, avoid reduction of available executors due to blacklist mechanism.
+
## Getting tablestatus.lock issues When loading data
**Symptom**
