From 8542fac9c29750818324fee1a6ce1a800e6c82c6 Mon Sep 17 00:00:00 2001
From: Richard Treu <richard.treu@sap.com>
Date: Thu, 25 Jan 2024 15:12:49 +0100
Subject: [PATCH] in_tail: Add option file_cache_advise to reduce file cache
 usage

This commit will add option file_cache_advise to the tail plugin.
It allows to set the posix_fadvise in POSIX_FADV_DONTNEED mode.
This will reduce the usage of the kernel file cache.

Feature is enabled by default.

Signed-off-by: Richard Treu <richard.treu@sap.com>
---
 plugins/in_tail/tail.c        |  8 +++++++-
 plugins/in_tail/tail_config.h |  3 +++
 plugins/in_tail/tail_file.c   | 17 ++++++++++++++++-
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/plugins/in_tail/tail.c b/plugins/in_tail/tail.c
index 8b37f5b3091..add73acfa43 100644
--- a/plugins/in_tail/tail.c
+++ b/plugins/in_tail/tail.c
@@ -696,7 +696,13 @@ static struct flb_config_map config_map[] = {
      0, FLB_TRUE, offsetof(struct flb_tail_config, skip_empty_lines),
      "Allows to skip empty lines."
     },
-
+#ifdef __linux__
+    {
+     FLB_CONFIG_MAP_BOOL, "file_cache_advise", "true",
+     0, FLB_TRUE, offsetof(struct flb_tail_config, file_cache_advise),
+     "Use posix_fadvise for file access. Advise not to use kernel file cache."
+    },
+#endif
 #ifdef FLB_HAVE_INOTIFY
     {
      FLB_CONFIG_MAP_BOOL, "inotify_watcher", "true",
diff --git a/plugins/in_tail/tail_config.h b/plugins/in_tail/tail_config.h
index ee02c3d05ea..2469e24cbe6 100644
--- a/plugins/in_tail/tail_config.h
+++ b/plugins/in_tail/tail_config.h
@@ -93,6 +93,9 @@ struct flb_tail_config {
     int   skip_long_lines;     /* skip long lines              */
     int   skip_empty_lines;    /* skip empty lines (off)       */
     int   exit_on_eof;         /* exit fluent-bit on EOF, test */
+#ifdef __linux__
+    int   file_cache_advise;   /* Use posix_fadvise for file access */
+#endif
 
     int progress_check_interval;      /* watcher interval             */
     int progress_check_interval_nsec; /* watcher interval             */
diff --git a/plugins/in_tail/tail_file.c b/plugins/in_tail/tail_file.c
index f681c7c3ad3..5a0937a59e4 100644
--- a/plugins/in_tail/tail_file.c
+++ b/plugins/in_tail/tail_file.c
@@ -940,7 +940,13 @@ int flb_tail_file_append(char *path, struct stat *st, int mode,
     if (flb_tail_file_exists(st, ctx) == FLB_TRUE) {
         return -1;
     }
-
+   
+    #ifdef __linux__
+    if (ctx->file_cache_advise) {
+        flb_plg_debug(ctx->ins, "file will be read in POSIX_FADV_DONTNEED mode %s", path);
+    }
+    #endif
+    
     fd = open(path, O_RDONLY);
     if (fd == -1) {
         flb_errno();
@@ -1429,6 +1435,15 @@ int flb_tail_file_chunk(struct flb_tail_file *file)
         file_buffer_capacity = (file->buf_size - file->buf_len) - 1;
     }
 
+    #ifdef __linux__
+    if (ctx->file_cache_advise) {
+        if (posix_fadvise(file->fd, 0, 0, POSIX_FADV_DONTNEED) == -1) {
+            flb_errno();
+            flb_plg_error(ctx->ins, "error during posix_fadvise");
+        }
+    }
+    #endif
+    
     read_size = file_buffer_capacity;
 
     if (file->decompression_context != NULL) {