fix code smell

diff --git a/metrics/interface/src/main/java/org/apache/iotdb/metrics/metricsets/jvm/JvmThreadMetrics.java b/metrics/interface/src/main/java/org/apache/iotdb/metrics/metricsets/jvm/JvmThreadMetrics.java
index 1dfcdcb..3ecf6f3 100644
--- a/metrics/interface/src/main/java/org/apache/iotdb/metrics/metricsets/jvm/JvmThreadMetrics.java
+++ b/metrics/interface/src/main/java/org/apache/iotdb/metrics/metricsets/jvm/JvmThreadMetrics.java

@@ -28,7 +28,7 @@
 import java.lang.management.ThreadInfo;
 import java.lang.management.ThreadMXBean;
 import java.util.Arrays;
-import java.util.HashMap;
+import java.util.EnumMap;
 import java.util.List;
 import java.util.Map;
 
@@ -36,8 +36,8 @@
 public class JvmThreadMetrics implements IMetricSet {
   private static long lastUpdateTime = 0L;
   private static final long UPDATE_INTERVAL = 10_000L;
-  private static Map<Thread.State, Integer> threadStateCountMap =
-      new HashMap<>(Thread.State.values().length + 1, 1.0f);
+  private static final Map<Thread.State, Integer> threadStateCountMap =
+      new EnumMap<>(Thread.State.class);
 
   @Override
   public void bindTo(AbstractMetricService metricService) {

diff --git a/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/FastCompactionPerformer.java b/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/FastCompactionPerformer.java
index d029122..9c45186 100644
--- a/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/FastCompactionPerformer.java
+++ b/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/FastCompactionPerformer.java

@@ -81,8 +81,6 @@
 
   private boolean isCrossCompaction;
 
-  private long tempFileSize = 0L;
-
   public FastCompactionPerformer(
       List<TsFileResource> seqFiles,
       List<TsFileResource> unseqFiles,

diff --git a/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/ReadPointCompactionPerformer.java b/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/ReadPointCompactionPerformer.java
index 8384916..f829974 100644
--- a/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/ReadPointCompactionPerformer.java
+++ b/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/performer/impl/ReadPointCompactionPerformer.java

@@ -75,7 +75,6 @@
   private CompactionTaskSummary summary;
 
   private List<TsFileResource> targetFiles = Collections.emptyList();
-  private long tempFileSize = 0L;
 
   public ReadPointCompactionPerformer(
       List<TsFileResource> seqFiles,

diff --git a/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/utils/executor/readchunk/AlignedSeriesCompactionExecutor.java b/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/utils/executor/readchunk/AlignedSeriesCompactionExecutor.java
index 2ec407a..ce38095 100644
--- a/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/utils/executor/readchunk/AlignedSeriesCompactionExecutor.java
+++ b/server/src/main/java/org/apache/iotdb/db/engine/compaction/execute/utils/executor/readchunk/AlignedSeriesCompactionExecutor.java

@@ -131,7 +131,6 @@
   }
 
   public void execute() throws IOException {
-    long originTempFileSize = writer.getPos();
     while (readerAndChunkMetadataList.size() > 0) {
       Pair<TsFileSequenceReader, List<AlignedChunkMetadata>> readerListPair =
           readerAndChunkMetadataList.removeFirst();

diff --git a/server/src/main/java/org/apache/iotdb/db/service/metrics/FileMetrics.java b/server/src/main/java/org/apache/iotdb/db/service/metrics/FileMetrics.java
index 563627f..cf590e1 100644
--- a/server/src/main/java/org/apache/iotdb/db/service/metrics/FileMetrics.java
+++ b/server/src/main/java/org/apache/iotdb/db/service/metrics/FileMetrics.java

@@ -35,6 +35,7 @@
 import org.slf4j.LoggerFactory;
 
 import java.io.BufferedReader;
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStreamReader;
 
@@ -48,6 +49,13 @@
   private final Runtime runtime = Runtime.getRuntime();
   private String[] getOpenFileNumberCommand;
 
+  @SuppressWarnings("squid:S1075")
+  private String fileHandlerCntPathInLinux = "/proc/%s/fd";
+
+  public FileMetrics() {
+    fileHandlerCntPathInLinux = String.format(fileHandlerCntPathInLinux, METRIC_CONFIG.getPid());
+  }
+
   @Override
   public void bindTo(AbstractMetricService metricService) {
     bindTsFileMetrics(metricService);
@@ -245,9 +253,17 @@
   }
 
   private long getOpenFileHandlersNumber() {
+    long fdCount = 0;
     try {
-      if ((METRIC_CONFIG.getSystemType() == SystemType.LINUX
-              || METRIC_CONFIG.getSystemType() == SystemType.MAC)
+      if (METRIC_CONFIG.getSystemType() == SystemType.LINUX) {
+        // count the fd in the system directory instead of
+        // calling runtime.exec() which could be much slower
+        File fdDir = new File(fileHandlerCntPathInLinux);
+        if (fdDir.exists()) {
+          File[] fds = fdDir.listFiles();
+          fdCount = fds == null ? 0 : fds.length;
+        }
+      } else if ((METRIC_CONFIG.getSystemType() == SystemType.MAC)
           && METRIC_CONFIG.getPid().length() != 0) {
         Process process = runtime.exec(getOpenFileNumberCommand);
         StringBuilder result = new StringBuilder();