content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package org.apache.bookkeeper.util;
  
  import java.io.FileDescriptor;
  import java.lang.reflect.Field;
  import lombok.experimental.UtilityClass;
  import lombok.extern.slf4j.Slf4j;
  import org.apache.bookkeeper.common.util.nativeio.NativeIO;
  import org.apache.bookkeeper.common.util.nativeio.NativeIOImpl;
  
  /**
   * Native I/O operations.
   */
  @UtilityClass
  @Slf4j
  public final class PageCacheUtil {
  
      private static final int POSIX_FADV_DONTNEED = 4; /* fadvise.h */
  
      private static boolean fadvisePossible = true;
  
      private static final NativeIO NATIVE_IO;
  
      static {
          NativeIO nativeIO = null;
          try {
              nativeIO = new NativeIOImpl();
          } catch (Exception e) {
              log.warn("Unable to initialize NativeIO for posix_fdavise: {}", e.getMessage());
              fadvisePossible = false;
          }
  
          NATIVE_IO = nativeIO;
      }
  
      private static Field getFieldByReflection(Class cls, String fieldName) {
          Field field = null;
  
          try {
              field = cls.getDeclaredField(fieldName);
              field.setAccessible(true);
          } catch (Exception e) {
              // We don't really expect this so throw an assertion to
              // catch this during development
              log.warn("Unable to read {} field from {}", fieldName, cls.getName());
              assert false;
          }
  
          return field;
      }
      /**
       * Get system file descriptor (int) from FileDescriptor object.
       * @param descriptor - FileDescriptor object to get fd from
       * @return file descriptor, -1 or error
       */
      public static int getSysFileDescriptor(FileDescriptor descriptor) {
          Field field = getFieldByReflection(descriptor.getClass(), "fd");
          try {
              return field.getInt(descriptor);
          } catch (Exception e) {
              log.warn("Unable to read fd field from java.io.FileDescriptor");
          }
  
          return -1;
      }
  
      /**
       * Remove pages from the file system page cache when they won't
       * be accessed again.
       *
       * @param fd     The file descriptor of the source file.
       * @param offset The offset within the file.
       * @param len    The length to be flushed.
       */
      public static void bestEffortRemoveFromPageCache(int fd, long offset, long len) {
          if (!fadvisePossible || fd < 0) {
              return;
          }
          try {
              NATIVE_IO.posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
          } catch (Throwable e) {
              log.warn("Failed to perform posix_fadvise: {}", e.getMessage());
              fadvisePossible = false;
          }
      }
  }
  