diff --git a/src/one/nio/os/Cpus.java b/src/one/nio/os/Cpus.java
index 19c9e726..6455a8dc 100644
--- a/src/one/nio/os/Cpus.java
+++ b/src/one/nio/os/Cpus.java
@@ -16,41 +16,43 @@
 
 package one.nio.os;
 
-import java.io.IOException;
-import java.nio.file.Files;
-import java.nio.file.Paths;
+import one.nio.util.Utf8;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
-import one.nio.util.Utf8;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.BitSet;
 
 public class Cpus {
     private static final Log log = LogFactory.getLog(Cpus.class);
 
-    public static final int ONLINE = cpus("/sys/devices/system/cpu/online");
-    public static final int POSSIBLE = cpus("/sys/devices/system/cpu/possible");
-    public static final int PRESENT = cpus("/sys/devices/system/cpu/present");
-
-    private static int cpus(String rangeFile) {
+    public static final BitSet ONLINE = cpus("/sys/devices/system/cpu/online");
+    public static final BitSet PRESENT = cpus("/sys/devices/system/cpu/present");
+    public static final BitSet POSSIBLE = cpus("/sys/devices/system/cpu/possible");
+    public static final int COUNT = POSSIBLE.cardinality();
+    
+    private static BitSet cpus(String rangeFile) {
         try {
             byte[] bytes = Files.readAllBytes(Paths.get(rangeFile));
             String rangeStr = Utf8.read(bytes, 0, bytes.length).trim();
-            int cpus = 0;
+            BitSet cpus = new BitSet();
             for (String range : rangeStr.split(",")) {
                 String[] s = range.split("-");
-                if (s.length == 1) {
-                    cpus++;
-                } else {
-                    cpus += 1 + Integer.parseInt(s[1]) - Integer.parseInt(s[0]);
-                }
+                int from = Integer.parseInt(s[0]);
+                int to = s.length == 1 ? from : Integer.parseInt(s[1]);
+                cpus.set(from, to);
             }
             return cpus;
         } catch (IOException e) {
             if (log.isDebugEnabled()) {
                 log.debug("Failed to read " + rangeFile, e);
             }
-            return Runtime.getRuntime().availableProcessors();
+            BitSet cpus = new BitSet();
+            cpus.set(0, Runtime.getRuntime().availableProcessors());
+            return cpus;
         }
     }
 }
diff --git a/src/one/nio/os/bpf/BpfMap.java b/src/one/nio/os/bpf/BpfMap.java
index c29632fe..23c15c63 100644
--- a/src/one/nio/os/bpf/BpfMap.java
+++ b/src/one/nio/os/bpf/BpfMap.java
@@ -28,7 +28,7 @@
 import java.util.NoSuchElementException;
 
 public class BpfMap extends BpfObj implements Closeable {
-    public static final int CPUS = Cpus.POSSIBLE;
+    public static final int CPUS = Cpus.COUNT;
     public static final int ARRAY_KEY_SIZE = 4;
 
     public final MapType type;
diff --git a/src/one/nio/os/native/perf.c b/src/one/nio/os/native/perf.c
index 9a18d8d9..2eda59c4 100644
--- a/src/one/nio/os/native/perf.c
+++ b/src/one/nio/os/native/perf.c
@@ -111,10 +111,6 @@ Java_one_nio_os_perf_Perf_openEvent(JNIEnv* env, jclass cls, jint pid, jint cpu,
         attr.config = config;
     }
 
-    if (type == PERF_TYPE_SOFTWARE) {
-        attr.precise_ip = 2;
-    }
-
     unsigned long flags = parse_perf_options(env, options, &attr);
 
     int fd = syscall(__NR_perf_event_open, &attr, pid, cpu, group, flags);
@@ -180,5 +176,7 @@ Java_one_nio_os_perf_Perf_getValue(JNIEnv* env, jclass cls, jint fd, jlongArray
 
 JNIEXPORT void JNICALL
 Java_one_nio_os_perf_Perf_ioctl(JNIEnv* env, jclass cls, jint fd, jint cmd, jint arg) {
-    ioctl(fd, ioctl_cmd[cmd], arg);
+    if (ioctl(fd, ioctl_cmd[cmd], arg) < 0) {
+        throw_io_exception(env);
+    }
 }
diff --git a/src/one/nio/os/perf/LocalValue.java b/src/one/nio/os/perf/LocalValue.java
index d250fbf1..6a93e081 100644
--- a/src/one/nio/os/perf/LocalValue.java
+++ b/src/one/nio/os/perf/LocalValue.java
@@ -17,6 +17,8 @@
 package one.nio.os.perf;
 
 public class LocalValue implements CounterValue {
+    public static final LocalValue ZERO = new LocalValue(0, 0, 0);
+    
     public final long value;
     public final long running;
     public final long enabled;
diff --git a/src/one/nio/os/perf/Perf.java b/src/one/nio/os/perf/Perf.java
index 58236597..0bd2d719 100644
--- a/src/one/nio/os/perf/Perf.java
+++ b/src/one/nio/os/perf/Perf.java
@@ -58,13 +58,15 @@ public static PerfCounter open(PerfEvent event, String cgroup, int cpu, PerfOpti
 
     public static PerfCounterGlobal openGlobal(PerfEvent event, int pid, PerfOption... options) throws IOException {
         String optionString = optionString(options);
-        int[] fds = new int[Cpus.PRESENT];
+        int[] fds = new int[Cpus.COUNT];
 
         PerfOptionGlobalGroup group = (PerfOptionGlobalGroup) option(options, PerfOption.GROUP_GLOBAL);
         try {
             for (int cpu = 0; cpu < fds.length; cpu++) {
-                int groupFd = group == null ? -1 : group.fds[cpu];
-                fds[cpu] = openEvent(pid, cpu, event.type, event.config, event.breakpoint, groupFd, optionString);
+                if (Cpus.ONLINE.get(cpu)) {
+                    int groupFd = group == null ? -1 : group.fds[cpu];
+                    fds[cpu] = openEvent(pid, cpu, event.type, event.config, event.breakpoint, groupFd, optionString);
+                }
             }
         } catch (Throwable e) {
             for (int fd : fds) {
diff --git a/src/one/nio/os/perf/PerfCounterGlobal.java b/src/one/nio/os/perf/PerfCounterGlobal.java
index 70039fea..dec434d1 100644
--- a/src/one/nio/os/perf/PerfCounterGlobal.java
+++ b/src/one/nio/os/perf/PerfCounterGlobal.java
@@ -35,7 +35,9 @@ public void close() {
             for (int i = 0; i < fds.length; i++) {
                 int fd = fds[i];
                 fds[i] = -1;
-                Perf.close(fd);
+                if (fd > 0) {
+                    Perf.close(fd);
+                }
             }
         }
     }
@@ -44,7 +46,9 @@ public void close() {
     public long get() throws IOException {
         long sum = 0;
         for (int fd : fds) {
-            sum += Perf.get(fd);
+            if (fd > 0) {
+                sum += Perf.get(fd);
+            }
         }
         return sum;
     }
@@ -54,18 +58,25 @@ public CounterValue getValue() throws IOException {
         int vals = hasReadFormat(ReadFormat.TOTAL_TIME_RUNNING | ReadFormat.TOTAL_TIME_ENABLED) ? 3 : 1;
         long[] buf = new long[3 * fds.length];
         for (int cpu = 0; cpu < fds.length; cpu++) {
-            Perf.getValue(fds[cpu], buf, cpu * 3, vals);
+            int fd = fds[cpu];
+            if (fd > 0) {
+                Perf.getValue(fd, buf, cpu * 3, vals);
+            }
         }
         return new GlobalValue(buf);
     }
 
     public long getForCpu(int cpu) throws IOException {
-        return Perf.get(fds[cpu]);
+        int fd = fds[cpu];
+        return fd > 0 ? Perf.get(fd) : 0;
     }
 
     public LocalValue getValueForCpu(int cpu) throws IOException {
+        int fd = fds[cpu];
+        if (fd <= 0) return LocalValue.ZERO;
+        
         long[] buf = newBuffer();
-        Perf.getValue(fds[cpu], buf, 0, buf.length);
+        Perf.getValue(fd, buf, 0, buf.length);
         return toValue(buf);
     }
 
@@ -74,9 +85,11 @@ protected long[] getRawValue() throws IOException {
         long[] buf = newBuffer();
         long[] total = newBuffer();
         for (int fd : fds) {
-            Perf.getValue(fd, buf, 0, buf.length);
-            for (int i = 0; i < buf.length; i++) {
-                total[i] += buf[i];
+            if (fd > 0) {
+                Perf.getValue(fd, buf, 0, buf.length);
+                for (int i = 0; i < buf.length; i++) {
+                    total[i] += buf[i];
+                }
             }
         }
         return total;
@@ -85,7 +98,9 @@ protected long[] getRawValue() throws IOException {
     @Override
     void ioctl(int cmd, int arg) throws IOException {
         for (int fd : fds) {
-            Perf.ioctl(fd, cmd, arg);
+            if (fd > 0) {
+                Perf.ioctl(fd, cmd, arg);
+            }
         }
     }
 
@@ -101,6 +116,9 @@ public void storeTo(BpfMap map, int cpu) throws IOException {
             throw new IllegalArgumentException();
         }
 
-        map.put(BpfMap.bytes(cpu), BpfMap.bytes(fds[cpu]));
+        int fd = fds[cpu];
+        if (fd > 0) {
+            map.put(BpfMap.bytes(cpu), BpfMap.bytes(fd));
+        }
     }
 }