liblttdvfs: put fadvise hint after sync writeback

[ltt-control.git] / liblttd / liblttdvfs.c

diff --git a/liblttd/liblttdvfs.c b/liblttd/liblttdvfs.c
index 6e28c9bd6937ebbe86c20b4e3c4a9cb1f871fa84..74a5766fa5dc5f8eae85f0aadf9cd91be63a31fb 100644 (file)
--- a/liblttd/liblttdvfs.c
+++ b/liblttd/liblttdvfs.c
@@ -185,21 +185,6 @@ int liblttdvfs_on_read_subbuffer(struct liblttd_callbacks *data, struct fd_pair
                        goto write_end;
                }
                len -= ret;
-               /*
-                * Give hints to the kernel about how we access the file:
-                * POSIX_FADV_DONTNEED : we won't re-access data in a near
-                * future after we write it.
-                * We need to call fadvise again after the file grows because
-                * the kernel does not seem to apply fadvise to non-existing
-                * parts of the file.
-                */
-               ret = posix_fadvise(outfd, 0, 0, POSIX_FADV_DONTNEED);
-               if (ret != 0) {
-                       perror("fadvise");
-                       /* Just a hint, not critical. Continue. */
-                       ret = 0;
-               }
-
                /* This won't block, but will start writeout asynchronously */
                sync_file_range(outfd, pair->offset, ret,
                                SYNC_FILE_RANGE_WRITE);
@@ -209,14 +194,31 @@ write_end:
        /*
         * This does a blocking write-and-wait on any page that belongs to the
         * subbuffer prior to the one we just wrote.
+        * Don't care about error values, as these are just hints and ways to
+        * limit the amount of page cache used.
         */
-       if (orig_offset >= pair->max_sb_size)
+       if (orig_offset >= pair->max_sb_size) {
                sync_file_range(outfd, orig_offset - pair->max_sb_size,
                                pair->max_sb_size,
                                SYNC_FILE_RANGE_WAIT_BEFORE
                                | SYNC_FILE_RANGE_WRITE
                                | SYNC_FILE_RANGE_WAIT_AFTER);
-       
+               /*
+                * Give hints to the kernel about how we access the file:
+                * POSIX_FADV_DONTNEED : we won't re-access data in a near
+                * future after we write it.
+                * We need to call fadvise again after the file grows because
+                * the kernel does not seem to apply fadvise to non-existing
+                * parts of the file.
+                * Call fadvise _after_ having waited for the page writeback to
+                * complete because the dirty page writeback semantic is not
+                * well defined. So it can be expected to lead to lower
+                * throughput in streaming.
+                */
+               posix_fadvise(outfd, orig_offset - pair->max_sb_size,
+                             pair->max_sb_size, POSIX_FADV_DONTNEED);
+       }
+
        return ret;
 }