OPTIM: halog: improve cold-cache behaviour when loading a file
Using posix_fadvise() it is possible to tell the system that we're
going to read a whole file at once. The kernel then doubles the
read-ahead size for this file. On Linux with an SSD, this has improved
cold-cache performance by around 20%. Hot-cache is not affected at all.
diff --git a/contrib/halog/halog.c b/contrib/halog/halog.c
index 250623d..448e5c0 100644
--- a/contrib/halog/halog.c
+++ b/contrib/halog/halog.c
@@ -672,6 +672,15 @@
else if (filter & FILT_COUNT_ONLY)
line_filter = NULL;
+#if defined(POSIX_FADV_SEQUENTIAL)
+ /* around 20% performance improvement is observed on Linux with this
+ * on cold-cache. Surprizingly, WILLNEED is less performant. Don't
+ * use NOREUSE as it flushes the cache and prevents easy data
+ * manipulation on logs!
+ */
+ posix_fadvise(0, 0, 0, POSIX_FADV_SEQUENTIAL);
+#endif
+
if (!line_filter &&
!(filter & (FILT_HTTP_ONLY|FILT_TIME_RESP|FILT_ERRORS_ONLY|FILT_HTTP_STATUS|FILT_QUEUE_ONLY|FILT_QUEUE_SRV_ONLY|FILT_TERM_CODE_NAME))) {
/* read the whole file at once first */